Fix ref as mutable ref in std::rc::Rc doc

Author: letheed

src/liballoc/rc.rs

@@ -273,7 +273,7 @@ struct RcBox<T: ?Sized> {
 /// See the [module-level documentation](./index.html) for more details.
 ///
 /// The inherent methods of `Rc` are all associated functions, which means
-/// that you have to call them as e.g. [`Rc::get_mut(&value)`][get_mut] instead of
+/// that you have to call them as e.g. [`Rc::get_mut(&mut value)`][get_mut] instead of
 /// `value.get_mut()`. This avoids conflicts with methods of the inner
 /// type `T`.
 ///