Clarify sentences about lock types. Fixes #2937.

src/ch16-03-shared-state.md

@@ -71,9 +71,10 @@ that case, no one would ever be able to get the lock, so we’ve chosen to
 
 After we’ve acquired the lock, we can treat the return value, named `num` in
 this case, as a mutable reference to the data inside. The type system ensures
-that we acquire a lock before using the value in `m`: `Mutex<i32>` is not an
-`i32`, so we *must* acquire the lock to be able to use the `i32` value. We
-can’t forget; the type system won’t let us access the inner `i32` otherwise.
+that we acquire a lock before using the value in `m`. The type of `m` is
+`Mutex<i32>`, not `i32`, so we *must* call `lock` to be able to use the `i32`
+value. We can’t forget; the type system won’t let us access the inner `i32`
+otherwise.
 
 As you might suspect, `Mutex<T>` is a smart pointer. More accurately, the call
 to `lock` *returns* a smart pointer called `MutexGuard`, wrapped in a