CRA 3 : Correct link to service worker doc

[codingdash]

Started from #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

[ianschmitz]

We are aware of the outdated bit.ly links (#5910). In the mean time would you like to submit a PR to update the link?

[codingdash]

@ianschmitz Here it goes #6668, please take a look at it.