Add omitted word to mutability docs.

Author: dpetersen

src/doc/trpl/mutability.md

@@ -78,8 +78,8 @@ When we call `clone()`, the `Arc<T>` needs to update the reference count. Yet
 we’ve not used any `mut`s here, `x` is an immutable binding, and we didn’t take
 `&mut 5` or anything. So what gives?
 
-To this, we have to go back to the core of Rust’s guiding philosophy, memory
-safety, and the mechanism by which Rust guarantees it, the
+To understand this, we have to go back to the core of Rust’s guiding
+philosophy, memory safety, and the mechanism by which Rust guarantees it, the
 [ownership][ownership] system, and more specifically, [borrowing][borrowing]:
 
 > You may have one or the other of these two kinds of borrows, but not both at