docs: make HeroService#gerHero() in toh-pt5 compatible with TS strict mode (angular#41234)

Fixes angular#41207

PR Close angular#41234

Author: gkalpak

aio/content/examples/toh-pt5/src/app/hero.service.ts

@@ -19,7 +19,9 @@ export class HeroService {
 
   // #docregion getHero
   getHero(id: number): Observable<Hero> {
-    const hero = HEROES.find(h => h.id === id);
+    // For now, assume that a hero with the specified `id` always exists.
+    // Error handling will be added in the next step of the tutorial.
+    const hero = HEROES.find(h => h.id === id) as Hero;
     this.messageService.add(`HeroService: fetched hero id=${id}`);
     return of(hero);
   }