message: docs(forms): Update catchError to return an `observable`
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diff: @@ -7,7 +7,7 @@ import {
  } from '@angular/forms';
  import { catchError, map } from 'rxjs/operators';
  import { HeroesService } from './heroes.service';
  -import { Observable } from 'rxjs';
  +import { Observable, of } from 'rxjs';
  // #docregion async-validator
  @Injectable({ providedIn: 'root' })
  @@ -19,7 +19,7 @@ export class UniqueAlterEgoValidator implements AsyncValidator {
  ): Promise<ValidationErrors | null> | Observable<ValidationErrors | null> {
  return this.heroesService.isAlterEgoTaken(ctrl.value).pipe(
  map(isTaken => (isTaken ? { uniqueAlterEgo: true } : null)),
  - catchError(() => null)
  + catchError(() => of(null))
  );
  }
  }
  