docs(router): mark title strategy as injectable (angular#46639)

Add the `@Injectable` decorator in the title strategy example so that it can be injected in the providers of the routing module.
PR Close angular#46639

Author: bampakoa

aio/content/examples/router/src/app/app-routing.module.10.ts

@@ -40,6 +40,7 @@ export class ResolvedChildATitle implements Resolve<string> {
 
 
 // #docregion custom-page-title
+@Injectable({providedIn: 'root'})
 export class TemplatePageTitleStrategy extends TitleStrategy {
   constructor(private readonly title: Title) {
     super();