message: docs: fix ch 4 so it compiles in strict mode
  Fixes error: `TS2564: Property 'heroes' has no initializer and is not definitely assigned in the constructor`
  and makes `hero` property consistent with ch 3 (ref:
  PR Close
diff: @@ -15,10 +15,10 @@ import { MessageService } from '../message.service';
  })
  export class HeroesComponent implements OnInit {
  - selectedHero: Hero;
  + selectedHero?: Hero;
  // #docregion heroes
  - heroes: Hero[];
  + heroes: Hero[] = [];
  // #enddocregion heroes
  constructor(private heroService: HeroService, private messageService: MessageService) { }
  