message: docs: fix typo in dependency injection guide
  ~~`HeroService` must provided in some parent injector~~
  *`HeroService` must be provided in some parent injector*
  PR Close
diff: @@ -162,7 +162,7 @@ Here's the revised component, making use of the injected service, side-by-side w
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  -`HeroService` must provided in some parent injector. The code in `HeroListComponent` doesn't depend on where `HeroService` comes from.
  +`HeroService` must be provided in some parent injector. The code in `HeroListComponent` doesn't depend on where `HeroService` comes from.
  If you decided to provide `HeroService` in `AppModule`, `HeroListComponent` wouldn't change.
  {@a singleton-services}
  