collecdtions: Add correct feature flag to example in docs

Author: Ulrik Sverdrup

src/libcollections/vec.rs

@@ -735,6 +735,8 @@ impl<T> Vec<T> {
     /// # Examples
     ///
     /// ```
+    /// # #![feature(collections)]
+    ///
     /// // Draining using `..` clears the whole vector.
     /// let mut v = vec![1, 2, 3];
     /// let u: Vec<_> = v.drain(..).collect();