message: docs(retry): add example for retry operator
diff: @@ -15,6 +15,36 @@ import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
  * during failed subscriptions. For example, if an Observable fails at first but emits [1, 2] then succeeds the second
  * time and emits: [1, 2, 3, 4, 5] then the complete stream of emissions and notifications
  * would be: [1, 2, 1, 2, 3, 4, 5, `complete`].
  + *
  + * ## Example
  + * ```ts
  + * import { interval, of, throwError } from 'rxjs';
  + * import { mergeMap, retry } from 'rxjs/operators';
  + *
  + * const source = interval(1000);
  + * const example = source.pipe(
  + * mergeMap(val => {
  + * if(val > 5){
  + * return throwError('Error!');
  + * }
  + * return of(val);
  + * }),
  + * //retry 2 times on error
  + * retry(2)
  + * );
  + *
  + * const subscribe = example.subscribe({
  + * next: val => console.log(val),
  + * error: val => console.log(`${val}: Retried 2 times then quit!`)
  + * });
  + *
  + * // Output:
  + * // 0..1..2..3..4..5..
  + * // 0..1..2..3..4..5..
  + * // 0..1..2..3..4..5..
  + * // "Error!: Retried 2 times then quit!"
  + * ```
  + *
  * @param {number} count - Number of retry attempts before failing.
  * @return {Observable} The source Observable modified with the retry logic.
  * @method retry
  