fix typo in {Rc, Arc}::get_mut_unchecked docs

Author: y21

library/alloc/src/rc.rs

@@ -1092,7 +1092,7 @@ impl<T: ?Sized> Rc<T> {
     /// # Safety
     ///
     /// If any other `Rc` or [`Weak`] pointers to the same allocation exist, then
-    /// they must be must not be dereferenced or have active borrows for the duration
+    /// they must not be dereferenced or have active borrows for the duration
     /// of the returned borrow, and their inner type must be exactly the same as the
     /// inner type of this Rc (including lifetimes). This is trivially the case if no
     /// such pointers exist, for example immediately after `Rc::new`.

library/alloc/src/sync.rs

@@ -1733,7 +1733,7 @@ impl<T: ?Sized> Arc<T> {
     /// # Safety
     ///
     /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
-    /// they must be must not be dereferenced or have active borrows for the duration
+    /// they must not be dereferenced or have active borrows for the duration
     /// of the returned borrow, and their inner type must be exactly the same as the
     /// inner type of this Rc (including lifetimes). This is trivially the case if no
     /// such pointers exist, for example immediately after `Arc::new`.