put feature attribute in example

Author: ashivaram23

library/alloc/src/vec/mod.rs

@@ -3011,6 +3011,8 @@ impl<T, A: Allocator> Vec<T, A> {
     /// # Examples
     ///
     /// ```
+    /// #![feature(vec_into_chunks)]
+    ///
     /// let vec = vec![0, 1, 2, 3, 4, 5, 6, 7];
     /// assert_eq!(vec.into_chunks::<3>(), [[0, 1, 2], [3, 4, 5]]);
     ///