Add missing mut to pin.rs docs

library/core/src/pin.rs

@@ -831,15 +831,13 @@
 //!     <code>fn get_pin_mut(self: [Pin]<[`&mut Self`]>) -> [Pin]<[`&mut T`]></code>.
 //!     Then we could do the following:
 //!     ```compile_fail
-//!     # use std::cell::RefCell;
-//!     # use std::pin::Pin;
-//!     fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
+//!     fn exploit_ref_cell<T>(mut rc: Pin<&mut RefCell<T>>) {
 //!         // Here we get pinned access to the `T`.
 //!         let _: Pin<&mut T> = rc.as_mut().get_pin_mut();
 //!
 //!         // And here we have `&mut T` to the same data.
 //!         let shared: &RefCell<T> = rc.into_ref().get_ref();
-//!         let borrow = shared.borrow_mut();
+//!         let mut borrow = shared.borrow_mut();
 //!         let content = &mut *borrow;
 //!     }
 //!     ```