docs: improve sample code in 0 - install.md

[kenjis]

Actually the class has the following use:

use CodeIgniter\Config\BaseConfig;
use CodeIgniter\Shield\Authentication\Actions\ActionInterface;
use CodeIgniter\Shield\Authentication\AuthenticatorInterface;
use CodeIgniter\Shield\Authentication\Authenticators\AccessTokens;
use CodeIgniter\Shield\Authentication\Authenticators\Session;
use CodeIgniter\Shield\Authentication\Passwords\ValidatorInterface;
use CodeIgniter\Shield\Models\UserModel;
use CodeIgniter\Shield\Config\Auth as ShieldAuth;