Add a tracking issue

src/libcollections/binary_heap.rs

@@ -254,7 +254,7 @@ impl<'a, T: Ord> DerefMut for PeekMut<'a, T> {
 
 impl<'a, T: Ord> PeekMut<'a, T> {
     /// Removes the peeked value from the heap and returns it.
-    #[unstable(feature = "binary_heap_peek_mut_pop", issue = "0")]
+    #[unstable(feature = "binary_heap_peek_mut_pop", issue = "38863")]
     pub fn pop(mut this: PeekMut<'a, T>) -> T {
         let value = this.heap.pop().unwrap();
         this.sift = false;