docs(router): add RxJS filter pipe operator to example (#25686)

This example for Angular 6 requires piping the RxJS filter operator in order to traverse the multitude of Router Events.

PR Close #25686

packages/router/src/events.ts

@@ -33,7 +33,9 @@ export type NavigationTrigger = 'imperative' | 'popstate' | 'hashchange';
  * ```
  * class MyService {
  *   constructor(public router: Router, logger: Logger) {
- *     router.events.filter(e => e instanceof RouterEvent).subscribe(e => {
+ *     router.events.pipe(
+ *       filter(e => e instanceof RouterEvent)
+ *     ).subscribe(e => {
  *       logger.log(e.id, e.url);
  *     });
  *   }