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/architecture/src/app/mini-app.ts

@@ -12,7 +12,7 @@ import { Component } from '@angular/core';
 // #enddocregion import-core-component
 
 @Component({
- selector: 'my-app',
+ selector: 'app-root',
  template: 'Welcome to Angular'
 })
 export class AppComponent {