message: docs(readme): fix number version in readme
diff: @@ -20,7 +20,7 @@ Reactive Extensions Library for JavaScript. This is a rewrite of [Reactive-Exten
  ## Versions In This Repository
  - [master](https://github.com/ReactiveX/rxjs/commits/master) - This is all of the current work, which is against v8 of RxJS right now
  -- [6.x](https://github.com/ReactiveX/rxjs/tree/7.x) - This is the branch for version 7.X
  +- [7.x](https://github.com/ReactiveX/rxjs/tree/7.x) - This is the branch for version 7.X
  - [6.x](https://github.com/ReactiveX/rxjs/tree/6.x) - This is the branch for version 6.X
  Most PRs should be made to **master**.
  