Update authentication.rst

Modified CustomPasswordHasher Sample code

* App::uses $className
   * Sample code for En and Ja are different
     * [En](https://github.com/cakephp/docs/blob/master/en/core-libraries/components/authentication.rst#creating-custom-password-hasher-classes)
     * [Ja](https://github.com/cakephp/docs/blob/master/ja/core-libraries/components/authentication.rst#%E3%82%AB%E3%82%B9%E3%82%BF%E3%83%A0%E3%83%91%E3%82%B9%E3%83%AF%E3%83%BC%E3%83%89%E3%83%8F%E3%83%83%E3%82%B7%E3%83%A5%E5%8C%96%E3%82%AF%E3%83%A9%E3%82%B9%E3%81%AE%E4%BD%9C%E6%88%90)  => Error occurs Ja...