Clarify wording in transmute example

* Change "four eights" to "four u8s"
* Change "a 32" to "a u32"

src/doc/book/casting-between-types.md

@@ -135,14 +135,14 @@ cast four bytes into a `u32`:
 ```rust,ignore
 let a = [0u8, 0u8, 0u8, 0u8];
 
-let b = a as u32; // four eights makes 32
+let b = a as u32; // four u8s makes a u32
 ```
 
 This errors with:
 
 ```text
 error: non-scalar cast: `[u8; 4]` as `u32`
-let b = a as u32; // four eights makes 32
+let b = a as u32; // four u8s makes a u32
         ^~~~~~~~
 ```