docs: improve RegisterMigration docs (#11225)

## Description

Small documentation improvement. By reading the function header I thought `forVersion` will mean the `destination` and made a bug. So, I propose to use more clear function argument name: `forVersion` -> `fromVersion`. 

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

simapp/app_test.go

@@ -106,7 +106,7 @@ func TestRunMigrations(t *testing.T) {
 	testCases := []struct {
 		name         string
 		moduleName   string
-		forVersion   uint64
+		fromVersion  uint64
 		expRegErr    bool // errors while registering migration
 		expRegErrMsg string
 		expRunErr    bool // errors while running migration
@@ -134,7 +134,7 @@ func TestRunMigrations(t *testing.T) {
 			false, "", false, "", 1,
 		},
 		{
-			"cannot register migration handler for same module & forVersion",
+			"cannot register migration handler for same module & fromVersion",
 			"bank", 1,
 			true, "another migration for module bank and version 1 already exists: internal logic error", false, "", 0,
 		},
@@ -151,8 +151,8 @@ func TestRunMigrations(t *testing.T) {
 			called := 0
 
 			if tc.moduleName != "" {
-				// Register migration for module from version `forVersion` to `forVersion+1`.
+				// Register migration for module from version `fromVersion` to `fromVersion+1`.
-				err = app.configurator.RegisterMigration(tc.moduleName, tc.forVersion, func(sdk.Context) error {
+				err = app.configurator.RegisterMigration(tc.moduleName, tc.fromVersion, func(sdk.Context) error {
 					called++
 
 					return nil

types/module/configurator.go

@@ -26,14 +26,14 @@ type Configurator interface {
 
 	// RegisterMigration registers an in-place store migration for a module. The
 	// handler is a migration script to perform in-place migrations from version
-	// `forVersion` to version `forVersion+1`.
+	// `fromVersion` to version `fromVersion+1`.
 	//
 	// EACH TIME a module's ConsensusVersion increments, a new migration MUST
 	// be registered using this function. If a migration handler is missing for
 	// a particular function, the upgrade logic (see RunMigrations function)
 	// will panic. If the ConsensusVersion bump does not introduce any store
 	// changes, then a no-op function must be registered here.
-	RegisterMigration(moduleName string, forVersion uint64, handler MigrationHandler) error
+	RegisterMigration(moduleName string, fromVersion uint64, handler MigrationHandler) error
 }
 
 type configurator struct {
@@ -41,7 +41,7 @@ type configurator struct {
 	msgServer   grpc.Server
 	queryServer grpc.Server
 
-	// migrations is a map of moduleName -> forVersion -> migration script handler
+	// migrations is a map of moduleName -> fromVersion -> migration script handler
 	migrations map[string]map[uint64]MigrationHandler
 }
 
@@ -68,8 +68,8 @@ func (c configurator) QueryServer() grpc.Server {
 }
 
 // RegisterMigration implements the Configurator.RegisterMigration method
-func (c configurator) RegisterMigration(moduleName string, forVersion uint64, handler MigrationHandler) error {
+func (c configurator) RegisterMigration(moduleName string, fromVersion uint64, handler MigrationHandler) error {
-	if forVersion == 0 {
+	if fromVersion == 0 {
 		return sdkerrors.Wrap(sdkerrors.ErrInvalidVersion, "module migration versions should start at 1")
 	}
 
@@ -77,11 +77,11 @@ func (c configurator) RegisterMigration(moduleName string, forVersion uint64, ha
 		c.migrations[moduleName] = map[uint64]MigrationHandler{}
 	}
 
-	if c.migrations[moduleName][forVersion] != nil {
+	if c.migrations[moduleName][fromVersion] != nil {
-		return sdkerrors.Wrapf(sdkerrors.ErrLogic, "another migration for module %s and version %d already exists", moduleName, forVersion)
+		return sdkerrors.Wrapf(sdkerrors.ErrLogic, "another migration for module %s and version %d already exists", moduleName, fromVersion)
 	}
 
-	c.migrations[moduleName][forVersion] = handler
+	c.migrations[moduleName][fromVersion] = handler
 
 	return nil
 }