Update short link for PWA (facebook#3872)

Author: bondz

template/src/index.js

@@ -8,5 +8,5 @@ ReactDOM.render(<App />, document.getElementById('root'));
 
 // If you want your app to work offline and load faster, you can change
 // unregister() to register() below. Note this comes with some pitfalls.
-// Learn more about service workers: http://bit.ly/2vJdu84
+// Learn more about service workers: http://bit.ly/CRA-PWA
 serviceWorker.unregister();