docs: fix issue angular#40941

setting the `hero` property as an optional property fixes the compilation
error: `Property 'hero' has no initializer and is not definitely assigned
in the constructor` when having the ts transpiler set to "strict" mode.

aio/content/examples/toh-pt3/src/app/hero-detail/hero-detail.component.ts

@@ -13,7 +13,7 @@ import { Hero } from '../hero';
 })
 export class HeroDetailComponent implements OnInit {
   // #docregion input-hero
-  @Input() hero: Hero;
+  @Input() hero?: Hero;
   // #enddocregion input-hero
 
   constructor() { }