Fix Slide reactive-forms-modifying-the-view

[sonukapoor]

Fix code to use the correct variable name for the input. It's called textEntryFormControl instead of textEntryControl

<form [formGroup]="textEntryForm" (ngSubmit)="addToDo()">
  <p>
    Item: <input [formControl]="textEntryControl">
    <button type="submit">Add</button>
  </p>
</form>