SPDX (#370)

packages/example-react/src/index.tsx

@@ -1,6 +1,5 @@
 // Copyright 2017-2020 @polkadot/example-react authors & contributors
-// This software may be modified and distributed under the terms
-// of the Apache-2.0 license. See the LICENSE file for details.
+// SPDX-License-Identifier: Apache-2.0
 
 import React, { useCallback, useEffect, useState } from 'react';
 import ReactDOM from 'react-dom';