Update src/ch04-03-slices.md for formatting

Author: chriskrycho

src/ch04-03-slices.md

@@ -16,12 +16,12 @@ slices, to understand the problem that slices will solve:
 fn first_word(s: &String) -> ?
 ```
 
-The first_word function has a &String as a parameter. We don’t need ownership, so this is fine. 
-(In idiomatic Rust, functions do not take ownership of their arguments unless they need to, 
-and the reasons for that will become clear as we keep going!)
-But what should we return? We don’t really have a way to talk about part of a string.
-However, we could return the index of the end of the word, indicated by a space.
-Let’s try that, as shown in Listing 4-7.
+The `first_word` function has a `&String` as a parameter. We don’t need
+ownership, so this is fine. (In idiomatic Rust, functions do not take ownership
+of their arguments unless they need to, and the reasons for that will become
+clear as we keep going!) But what should we return? We don’t really have a way
+to talk about part of a string. However, we could return the index of the end of
+the word, indicated by a space. Let’s try that, as shown in Listing 4-7.
 
 <Listing number="4-7" file-name="src/main.rs" caption="The `first_word` function that returns a byte index value into the `String` parameter">