docs: use `ValidationErrors` interface in Form validation examples (#42135)

PR Close #42135

aio/content/examples/form-validation/src/app/shared/forbidden-name.directive.ts

@@ -5,7 +5,7 @@ import { AbstractControl, NG_VALIDATORS, ValidationErrors, Validator, ValidatorF
 // #docregion custom-validator
 /** A hero's name can't match the given regular expression */
 export function forbiddenNameValidator(nameRe: RegExp): ValidatorFn {
-  return (control: AbstractControl): {[key: string]: any} | null => {
+  return (control: AbstractControl): ValidationErrors | null => {
     const forbidden = nameRe.test(control.value);
     return forbidden ? {forbiddenName: {value: control.value}} : null;
   };