Fixed a syntax error in the pin docs

Author: Alex Gaynor

src/libcore/pin.rs

@@ -215,7 +215,7 @@
 //!     had a method `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`.
 //!     Then we could do the following:
 //!     ```compile_fail
-//!     fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>) {
+//!     fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) {
 //!         { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`.
 //!         let rc_shr: &RefCell<T> = rc.into_ref().get_ref();
 //!         let b = rc_shr.borrow_mut();