Update custom_password_authenticator.rst

Correct me if I am wrong... but because we are passing here ```username``` as first argument to ```UsernamePasswordToken```, when one would like to use method ```getUser()``` from base ```Controller```: 
```
...
if (!$this->container->has('security.context')) {
            throw new \LogicException('The SecurityBundle is not registered in your application.');
        }

        if (null === $token = $this->container->get('security.context')->getToken()) {
            return null;
        }

        if (!is_object($user = $token->getUser())) {
            return null;
        }

        return $user;
...
```
it will fail to get actually logged in user and return null.