correct comments in errors2.rs

exercises/error_handling/errors2.rs

@@ -2,7 +2,7 @@
 // Say we're writing a game where you can buy items with tokens. All items cost
 // 5 tokens, and whenever you purchase items there is a processing fee of 1
 // token. A player of the game will type in how many items they want to buy,
-// and the `total_cost` function will calculate the total number of tokens.
+// and the `total_cost` function will calculate the total cost of the tokens.
 // Since the player typed in the quantity, though, we get it as a string-- and
 // they might have typed anything, not just numbers!