Make docs for Vec::push() use vec! with square brackets

src/libcollections/vec.rs

@@ -581,7 +581,7 @@ pub fn retain<F>(&mut self, mut f: F) where F: FnMut(&T) -> bool {
     /// # Examples
     ///
     /// ```
-    /// let mut vec = vec!(1, 2);
+    /// let mut vec = vec![1, 2];
     /// vec.push(3);
     /// assert_eq!(vec, [1, 2, 3]);
     /// ```