Revert "docs(forms): use a number as input value for formControlName (#30606)" (#32088)

This reverts commit bed680cff8.

PR Close #32088

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>