book: Use \cdot for multiplications

Co-authored-by: Daira Hopwood <daira@jacaranda.org>

book/src/user/simple-example.md

@@ -4,7 +4,7 @@ Let's start with a simple circuit, to introduce you to the common APIs and how t
 used. The circuit will take a public input $c$, and will prove knowledge of two private
 inputs $a$ and $b$ such that
 
-$$a * b = c.$$
+$$a \cdot b = c.$$
 
 ```rust
 # extern crate halo2;

book/src/user/tips-and-tricks.md

@@ -12,11 +12,11 @@ In halo2 circuits, you can similarly constrain a cell to have one of a small set
 values. For example, to constrain $a$ to the range $[0..5]$, you would create a gate of
 the form:
 
-$$a * (1 - a) * (2 - a) * (3 - a) * (4 - a) = 0$$
+$$a \cdot (1 - a) \cdot (2 - a) \cdot (3 - a) \cdot (4 - a) = 0$$
 
 while to constraint $c$ to be either 7 or 13, you would use:
 
-$$(7 - c) * (13 - c) = 0$$
+$$(7 - c) \cdot (13 - c) = 0$$
 
 > The underlying principle here is that we create a polynomial constraint with roots at
 > each value in the set of possible values we want to allow. In R1CS circuits, the maximum