docs(contributing): fix typo in CONTRIBUTING.md (#4926)
docs(contributing): fix typo in CONTRIBUTING.md (#4926)

change usage of "work and progress" term, as it probably was meant to be
"work in progress"

closes #4925
benlesh/RxJS,benlesh/RxJS,kwonoj/RxJS,trxcllnt/RxJS,ReactiveX/rxjs,benlesh/RxJS,martinsik/rxjs,ReactiveX/RxJS,martinsik/rxjs,ReactiveX/rxjs,kwonoj/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,ReactiveX/rxjs,ReactiveX/RxJS,kwonoj/RxJS,benlesh/RxJS,trxcllnt/RxJS,kwonoj/RxJS,martinsik/rxjs,trxcllnt/RxJS,martinsik/rxjs,ReactiveX/RxJS,trxcllnt/RxJS