Fix Pin example from dangerous implicit autorefs

Author: Urgau

library/core/src/pin.rs

@@ -651,6 +651,7 @@
 //! [`Unmovable`] (from the example above), we could write an assignment function like so:
 //!
 //! ```
+//! # #![feature(slice_ptr_len)]
 //! # use std::pin::Pin;
 //! # use std::marker::PhantomPinned;
 //! # use std::ptr::NonNull;
@@ -676,7 +677,7 @@
 //!             let data_ptr = unpinned_src.data.as_ptr() as *const u8;
 //!             let slice_ptr = unpinned_src.slice.as_ptr() as *const u8;
 //!             let offset = slice_ptr.offset_from(data_ptr) as usize;
-//!             let len = (*unpinned_src.slice.as_ptr()).len();
+//!             let len = unpinned_src.slice.as_ptr().len();
 //!
 //!             unpinned_self.slice = NonNull::from(&mut unpinned_self.data[offset..offset+len]);
 //!         }