Update file header date in contributing.md (flutter#714)

Author: 0xdanny

CONTRIBUTING.md

@@ -123,7 +123,7 @@ reviewer.
 All files in the project must start with the following header.
 
 ```
-// Copyright 2020 The Flutter team. All rights reserved.
+// Copyright 2021 The Flutter team. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 ```