user_resource.php

<?php
public function toArray($request)
{
    return [
        'name' => $this->name,
        'email'=> $this->email,
        "created_at" => Carbon::createFromFormat('Y-m-d H:i:s', $this->created_at)->format('d-m-Y'),
        "invoices" => InvoicesResource::collection($this->whenLoaded('invoices')),
        "dni" => $this->dni
    ];
}