hide another #[allow] directive from a docs example

oconnor663 · oconnor663 · commit c1023e9e5f35 · 2022-04-08T10:29:50.000-07:00
This is a repeat for Rc of e0e64a8, which cleaned up the same thing for Arc.
diff --git a/library/alloc/src/rc.rs b/library/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 {
