Change type from u32 to u8 to match IPv4 spec

Each segment of an ipv4 address can only be 0-255, so each of the
segments only need to be u8s.

If, however, this was done to not have to explain the difference between
u32 and u8, or if this would be distracting from the point about enums
trying to be made here, I would be fine reverting this change.

src/ch06-01-enums.md

@@ -82,12 +82,12 @@ We can attach data to each variant of the enum directly. No need for an extra
 struct. But beyond that, this approach is better than using a struct alongside
 our enum because we can attatch different kinds of data to each variant.
 Imagine that instead of a `String`, we would prefer to store a `V4` as its four
-individual components, while leaving the `V6` variant as a `String`. With our
+individual components while leaving the `V6` variant as a `String`. With our
 struct, we’d be stuck. But enums deal with this case with ease:
 
 ```rust
 enum IpAddr {
-    V4(u32, u32, u32, u32),
+    V4(u8, u8, u8, u8),
     V6(String),
 }