facade应该返回类字符串，这样可以通过provider自定义

[augushong]

<?php

namespace think\captcha\facade;

use think\Facade;

/**
 * Class Captcha
 * @package think\captcha\facade
 * @mixin \think\captcha\Captcha
 */
class Captcha extends Facade
{
    protected static function getFacadeClass()
    {
        return 'think\captcha\Captcha';
    }
}