docs(subject.md): clarify that refCount is an operator (#4774)
docs(subject.md): clarify that refCount is an operator (#4774)

Clarify that refCount is an operator and not a method used directly on a ConnectableObservable.

Closes #4664
martinsik/rxjs,trxcllnt/RxJS,ReactiveX/rxjs,trxcllnt/RxJS,trxcllnt/RxJS,ReactiveX/RxJS,kwonoj/RxJS,ReactiveX/RxJS,martinsik/rxjs,benlesh/RxJS,benlesh/RxJS,ReactiveX/RxJS,kwonoj/RxJS,ReactiveX/rxjs,martinsik/rxjs,benlesh/RxJS,martinsik/rxjs,benlesh/RxJS,ReactiveX/rxjs,trxcllnt/RxJS,kwonoj/RxJS,ReactiveX/rxjs,kwonoj/RxJS,ReactiveX/RxJS