Fix a comment that was made stale before launch by #1016 (commit 542da61).

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

src/main.cpp

@@ -3093,11 +3093,10 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
         }
     }
 
-    // Enforce block.nVersion=2 rule that the coinbase starts with serialized block height
-    // if 750 of the last 1,000 blocks are version 2 or greater (51/100 if testnet):
-    // Since MIN_BLOCK_VERSION = 4 all blocks with nHeight > 0 should satisfy this.
-    // This rule is not applied to the genesis block, which didn't include the height
-    // in the coinbase.
+    // Enforce BIP 34 rule that the coinbase starts with serialized block height.
+    // In Zcash this has been enforced since launch, except that the genesis
+    // block didn't include the height in the coinbase (see Zcash protocol spec
+    // section '6.8 Bitcoin Improvement Proposals').
     if (nHeight > 0)
     {
         CScript expect = CScript() << nHeight;