Ignore partial code blocks, unindent them

src/ch04-03-slices.md

@@ -36,7 +36,7 @@ fn first_word(s: &String) -> usize {
 
 Let’s break that down a bit:
 
-```rust
+```rust,ignore
 let bytes = s.as_bytes();
 ```
 
@@ -44,7 +44,7 @@ Since we need to go through the String element by element and
 check if a value is a space, we will convert our String to an
 array of bytes using the `.as_bytes()` method.
 
-```rust
+```rust,ignore
 for (i, &byte) in bytes.iter().enumerate() {
 ```
 
@@ -59,7 +59,7 @@ Since it’s a tuple, we can use patterns, just like elsewhere in Rust.
 So we match against the tuple with i for the index and &byte for
 the byte itself.
 
-```rust
+```rust,ignore
     if byte == 32 {
         return i;
     }