Users API: Fixed incorrect created_at date on index endpoint

For #4325

app/Activity/Models/Activity.php

@@ -19,6 +19,8 @@ use Illuminate\Support\Str;
  * @property string $entity_type
  * @property int    $entity_id
  * @property int    $user_id
+ * @property Carbon $created_at
+ * @property Carbon $updated_at
  */
 class Activity extends Model
 {