Fix unclear wording in 4.3

Author: L0uisc

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 tries to take a mutable reference, 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, 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!
 
 #### String Literals Are Slices