docs(operators.md): fix grammar (#5636)
docs(operators.md): fix grammar (#5636)

Closes #5635
ReactiveX/RxJS,benlesh/RxJS,martinsik/rxjs,ReactiveX/RxJS,martinsik/rxjs,benlesh/RxJS,kwonoj/RxJS,ReactiveX/RxJS,ReactiveX/rxjs,benlesh/RxJS,ReactiveX/rxjs,kwonoj/RxJS,martinsik/rxjs,kwonoj/RxJS,ReactiveX/rxjs,benlesh/RxJS,martinsik/rxjs,ReactiveX/rxjs,kwonoj/RxJS,ReactiveX/RxJS