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/setup/src/app/app.component.ts

@@ -2,7 +2,7 @@
 import { Component } from '@angular/core';
 
 @Component({
-  selector: 'my-app',
+  selector: 'app-root',
   template: `<h1>Hello {{name}}</h1>`
 })
 export class AppComponent { name = 'Angular'; }