hide another #[allow] directive from a docs example

This is a repeat for Rc of 4b2ada814de7a5c1764af26ffe07dae547fd7b27,
which cleaned up the same thing for Arc.

Author: oconnor663

alloc/src/rc.rs

@@ -393,7 +393,7 @@ impl<T> Rc<T> {
     /// # Examples
     ///
     /// ```
-    /// #![allow(dead_code)]
+    /// # #![allow(dead_code)]
     /// use std::rc::{Rc, Weak};
     ///
     /// struct Gadget {