fix(forms): Broken link NG_VALIDATORS replace by an example (#15480)

PR Close #15480

packages/forms/src/validators.ts

@@ -24,6 +24,20 @@ function isEmptyInputValue(value: any): boolean {
  *
  * Provide this using `multi: true` to add validators.
  *
+ * ### Example
+ *
+ * ```typescript
+ * @Directive({
+ *   selector: '[custom-validator]',
+ *   providers: [{provide: NG_VALIDATORS, useExisting: CustomValidatorDirective, multi: true}]
+ * })
+ * class CustomValidatorDirective implements Validator {
+ *   validate(control: AbstractControl): ValidationErrors | null {
+ *     return {"custom": true};
+ *   }
+ * }
+ * ```
+ *
  * @stable
  */
 export const NG_VALIDATORS = new InjectionToken<Array<Validator|Function>>('NgValidators');