message: docs(windowToggle): fix example
diff: @@ -27,14 +27,14 @@ import { OperatorFunction } from '../types';
  * ## Example
  * Every other second, emit the click events from the next 500ms
  * ```javascript
  - * import { fromEvent, interval } from 'rxjs';
  + * import { fromEvent, interval, EMPTY } from 'rxjs';
  * import { windowToggle, mergeAll } from 'rxjs/operators';
  *
  * const clicks = fromEvent(document, 'click');
  * const openings = interval(1000);
  * const result = clicks.pipe(
  - * windowToggle(openings, i => i % 2 ? interval(500) : empty()),
  - * mergeAll(),
  + * windowToggle(openings, i => i % 2 ? interval(500) : EMPTY),
  + * mergeAll()
  * );
  * result.subscribe(x => console.log(x));
  * ```
  