docs(forms): use a number as input value for formControlName (#30606)

PR #29473 changed the docs to use a string as the input value of `formControlName`, as it used to only accept a string.
This has been changed, and `formControlName` now accepts a string or a number, so the example in the docs can use a binding as they used to.

PR Close #30606

packages/examples/forms/ts/nestedFormArray/nested_form_array_example.ts

@@ -17,7 +17,7 @@ import {FormArray, FormControl, FormGroup} from '@angular/forms';
     <form [formGroup]="form" (ngSubmit)="onSubmit()">
       <div formArrayName="cities">
         <div *ngFor="let city of cities.controls; index as i">
-          <input formControlName="{{i}}" placeholder="City">
+          <input [formControlName]="i" placeholder="City">
         </div>
       </div>
       <button>Submit</button>