Split up a long sentence

src/ch04-03-slices.md

@@ -284,9 +284,9 @@ error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immuta
 
 Recall from the borrowing rules that if we have an immutable reference to
 something, we cannot also take a mutable reference. Because `clear` needs to
-truncate the `String`, it needs to get a mutable reference, which the compiler
-disallows, which fails. Not only has Rust made our API easier to use, but it
-has also eliminated an entire class of errors at compile time!
+truncate the `String`, it needs to get a mutable reference. Rust disallows
+this, and compilation fails. Not only has Rust made our API easier to use, but
+it has also eliminated an entire class of errors at compile time!
 
 #### String Literals Are Slices