docs(marbles): fix sync group example (#3584)
docs(marbles): fix sync group example (#3584)

Closes #3581
ReactiveX/rxjs,benlesh/RxJS,trxcllnt/RxJS,benlesh/RxJS,trxcllnt/RxJS,trxcllnt/RxJS,martinsik/rxjs,Brocco/RxJS,benlesh/RxJS,kwonoj/RxJS,ReactiveX/RxJS,trxcllnt/RxJS,kwonoj/RxJS,ReactiveX/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,Brocco/RxJS,martinsik/rxjs,benlesh/RxJS,martinsik/rxjs,kwonoj/RxJS,ReactiveX/rxjs,kwonoj/RxJS,Brocco/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,Brocco/RxJS,martinsik/rxjs