Fix formatting.

Author: tomasz-rozanski

src/liballoc/collections/vec_deque.rs

@@ -1847,7 +1847,7 @@ impl<T> VecDeque<T> {
     ///
     /// let mut buf = VecDeque::new();
     /// buf.extend(1..5);
-    /// buf.retain(|&x| x%2 == 0);
+    /// buf.retain(|&x| x % 2 == 0);
     /// assert_eq!(buf, [2, 4]);
     /// ```
     ///