message: docs(forms): remove rxjs of alias import in form async validator example
  In the code example of the AsyncValidator example there was an aliased
  import for the rxjs operator `of`. To align with the RxJS docs it should
  just use a plain import of `of`
  PR Close
diff: @@ -81,7 +81,7 @@ export interface Validator {
  * async validator directive with a custom error key.
  *
  * ```typescript
  - * import { of as observableOf } from 'rxjs';
  + * import { of } from 'rxjs';
  *
  * @Directive({
  * selector: '[customAsyncValidator]',
  @@ -90,7 +90,7 @@ export interface Validator {
  * })
  * class CustomAsyncValidatorDirective implements AsyncValidator {
  * validate(control: AbstractControl): Observable<ValidationErrors|null> {
  - * return observableOf({'custom': true});
  + * return of({'custom': true});
  * }
  * }
  * ```
  