refactor: ensure all 'TODO's are consistent (#23252)

PR Close #23252

aio/content/examples/toh-pt4/src/app/hero.service.ts

@@ -26,7 +26,7 @@ export class HeroService {
   // #docregion getHeroes, getHeroes-1
   getHeroes(): Observable<Hero[]> {
     // #enddocregion getHeroes-1
-    // Todo: send the message _after_ fetching the heroes
+    // TODO: send the message _after_ fetching the heroes
     this.messageService.add('HeroService: fetched heroes');
     // #docregion getHeroes-1
     return of(HEROES);