Fix the missing line in the guessing-game.md

r? @steveklabnik

Author: vessd

src/doc/book/guessing-game.md

@@ -908,6 +908,7 @@ let guess: u32 = match guess.trim().parse() {
 ```
 
 This is how you generally move from ‘crash on error’ to ‘actually handle the
+error’, by switching from `expect()` to a `match` statement. The `Result`
 returned by `parse()` is an `enum`  like `Ordering`, but in this case, each
 variant has some data associated with it: `Ok` is a success, and `Err` is a
 failure. Each contains more information: the successfully parsed integer, or an