Update operators.md (#3221)
Update operators.md (#3221)

Cache was removed.
https://github.com/ReactiveX/rxjs/pull/2012
ReactiveX/rxjs,Brocco/RxJS,ReactiveX/RxJS,kwonoj/RxJS,jayphelps/rxjs,Brocco/RxJS,ReactiveX/rxjs,martinsik/rxjs,benlesh/RxJS,jayphelps/rxjs,IgorMinar/RxJS,kwonoj/RxJS,jayphelps/rxjs,benlesh/RxJS,trxcllnt/RxJS,trxcllnt/RxJS,kwonoj/RxJS,Brocco/RxJS,martinsik/rxjs,ReactiveX/RxJS,Brocco/RxJS,kwonoj/RxJS,benlesh/RxJS,jayphelps/rxjs,ReactiveX/rxjs,jayphelps/rxjs,martinsik/rxjs,trxcllnt/RxJS,ReactiveX/rxjs,benlesh/RxJS,trxcllnt/RxJS,IgorMinar/RxJS,martinsik/rxjs,ReactiveX/RxJS,ReactiveX/RxJS