message: docs(operators.md): fix grammar
  Closes
diff: @@ -301,7 +301,7 @@ import { Observable } from 'rxjs';
  function delay(delayInMillis) {
  return (observable) => new Observable(observer => {
  - // this function will called each time this
  + // this function will be called each time this
  // Observable is subscribed to.
  const allTimerIDs = new Set();
  const subscription = observable.subscribe({
  