docs: update example references from "my-app" to "app-root" (#30789)

"app" is the default prefix for CLI projects when generating components.
This updates our examples to conform that that default

Closes #19778

PR Close #30789

aio/content/examples/styleguide/src/01-01/app/heroes/hero.component.avoid.ts

@@ -10,7 +10,7 @@ class Hero {
 }
 
 @Component({
-  selector: 'my-app',
+  selector: 'app-root',
   template: `
       <h1>{{title}}</h1>
       <pre>{{heroes | json}}</pre>