message: doc: fix typo in features section associated 6.0.0-rc.0 changes
  Change 'sourec' to be 'source'.
diff: ### Features
  -* **takeUntil:** no longer subscribes to sourec if notifier synchronously emits ([#3504](https://github.com/ReactiveX/RxJS/issues/3504)) ([7b8a3e3](https://github.com/ReactiveX/RxJS/commit/7b8a3e3)), closes [#2189](https://github.com/ReactiveX/RxJS/issues/2189)
  +* **takeUntil:** no longer subscribes to source if notifier synchronously emits ([#3504](https://github.com/ReactiveX/RxJS/issues/3504)) ([7b8a3e3](https://github.com/ReactiveX/RxJS/commit/7b8a3e3)), closes [#2189](https://github.com/ReactiveX/RxJS/issues/2189)
  ### Performance Improvements
  