fix example

Author: antonilol

library/alloc/src/collections/vec_deque/mod.rs

@@ -1991,8 +1991,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// #![feature(deque_extend_front)]
     /// use std::collections::VecDeque;
     ///
-    /// let mut d = VecDeque::from([4, 5, 6]);
-    /// d.extend_front([3, 2, 1]);
+    /// let mut deque = VecDeque::from([4, 5, 6]);
+    /// deque.extend_front([3, 2, 1]);
     /// assert_eq!(deque, [1, 2, 3, 4, 5, 6]);
     /// ```
     #[unstable(feature = "deque_extend_front", issue = "none")]