Remove unnecessary mut in docs causing test failures.

Author: nathankleyn

src/libcollections/binary_heap.rs

@@ -660,7 +660,7 @@ impl<T: Ord> BinaryHeap<T> {
     ///
     /// ```
     /// use std::collections::BinaryHeap;
-    /// let mut heap = BinaryHeap::from(vec![1, 3]);
+    /// let heap = BinaryHeap::from(vec![1, 3]);
     ///
     /// assert_eq!(heap.len(), 2);
     /// ```