Removed serviceWorker.js from folder structure in README.md

Created by: PhoenixCreation

Removing serviceWorker.js which is not provided by default. Also added reportWebVitals.js which is now included.

- serviceWorker.js
+ reportWebVitals.js

I personally run into confusion while finding the service worker. Because generally all the docs about create-react-app says that serviceWorker.js is provided by default. But it is not the case now and showing that it should be available from first README.md makes confusion that 'why I don't have it?'.

I found someone mentioned this is issues but nothing happened so I am creating this PR. Thank you.