Update http_authentication.rst

The preferred way to get the container in functional tests is `self::$container` (see https://symfony.com/doc/current/testing.html#accessing-the-container).

The issue with `$this->client->getContainer()` is that `Symfony\Bundle\FrameworkBundle\KernelBrowser::getContainer()` can return `null`. This can lead to a NullPointerException when calling `get()` on it.

Author: thled

testing/http_authentication.rst

@@ -111,7 +111,7 @@ needs::
 
         private function logIn()
         {
-            $session = $this->client->getContainer()->get('session');
+            $session = self::$container->get('session');
 
             $firewallName = 'secure_area';
             // if you don't define multiple connected firewalls, the context defaults to the firewall name