message: docs: fixed typo in AsyncValidatorFn
  the return type must be
  Observable<ValidationErrors | null>
  instead of
  Observable<ValidationErrors> | null
  PR Close
diff: @@ -7,7 +7,7 @@ Async validators must return a promise or an observable, and emit/resolve them w
  ```typescript
  export function isTenAsync(control: AbstractControl):
  - Observable<ValidationErrors> | null {
  + Observable<ValidationErrors | null> {
  const v: number = control.value;
  if (v !== 10) {
  // Emit an object with a validation error.
  