fix(#2161): Update Angular DatePicker sandbox code

[ArakTaiRoth]

Before

Code under Date Picker Reactive Forms example:

import { FormControl } from "@angular/forms";
export class MyComponent {
  reactiveFormCtrl = new FormControl(new Date());
}

After

Code under Date Picker Reactive Forms example:

import { FormControl } from "@angular/forms";
export class MyComponent {
  itemFormCtrl = new FormControl(new Date());
}

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[lizhuomeng71]

issue is verified on angular