message: docs: remove beta tag since v7 is now official
diff: [![npm version](https://badge.fury.io/js/rxjs.svg)](http://badge.fury.io/js/rxjs)
  [![Join the chat at https://gitter.im/Reactive-Extensions/RxJS](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/Reactive-Extensions/RxJS?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
  -# RxJS 7 (beta)
  +# RxJS 7
  ### FOR 6.X PLEASE GO TO [THE 6.x BRANCH](https://github.com/ReactiveX/rxjs/tree/6.x)
  