consensus/ethash: fix a typo and error message (#20503)

Author: gballet

consensus/ethash/consensus.go

@@ -68,7 +68,7 @@ var (
 // codebase, inherently breaking if the engine is swapped out. Please put common
 // error types into the consensus package.
 var (
-	errZeroBlockTime     = errors.New("timestamp equals parent's")
+	errOlderBlockTime    = errors.New("timestamp older than parent")
 	errTooManyUncles     = errors.New("too many uncles")
 	errDuplicateUncle    = errors.New("duplicate uncle")
 	errUncleIsAncestor   = errors.New("uncle is ancestor")
@@ -255,9 +255,9 @@ func (ethash *Ethash) verifyHeader(chain consensus.ChainReader, header, parent *
 		}
 	}
 	if header.Time <= parent.Time {
-		return errZeroBlockTime
+		return errOlderBlockTime
 	}
-	// Verify the block's difficulty based in its timestamp and parent's difficulty
+	// Verify the block's difficulty based on its timestamp and parent's difficulty
 	expected := ethash.CalcDifficulty(chain, header.Time, parent)
 
 	if expected.Cmp(header.Difficulty) != 0 {