allow unused mut for pinning explanation

src/liballoc/pin.rs

@@ -77,6 +77,7 @@
 //! // The pointer should point to the correct location,
 //! // so long as the struct hasn't moved.
 //! // Meanwhile, we are free to move the pointer around.
+//! # #[allow(unused_mut)]
 //! let mut still_unmoved = unmoved;
 //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
 //!