CRA 3 : Correct link to service worker doc

Created by: codingdash

Started from https://github.com/facebook/create-react-app/issues/6475

In example src/index.js in following comment the link needs to be updated.

// 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: https://bit.ly/CRA-PWA
serviceWorker.unregister();

From https://bit.ly/CRA-PWA -> https://facebook.github.io/create-react-app/docs/making-a-progressive-web-app