Clarify hook name

Hook name used is ``getAuthenticationService``:
https://github.com/cakephp/authentication/blob/1.4.0/src/Middleware/AuthenticationMiddleware.php#L153

Author: ravage84

docs/en/index.rst

@@ -27,7 +27,8 @@ Add the authentication to the middleware. See the CakePHP `documentation
 <http://book.cakephp.org/3.0/en/controllers/middleware.html>`_ on how to use
 middleware if you are not familiar with it.
 
-Example of configuring the authentication middleware using ``authentication`` application hook::
+Example of configuring the authentication middleware using the ``getAuthenticationService``
+hook within ``Application``::
 
     use Authentication\AuthenticationService;
     use Authentication\AuthenticationServiceProviderInterface;