use a placeholder for the constructor arguments

Author: xabbuh

components/http_foundation/session_configuration.rst

@@ -83,7 +83,7 @@ Example usage::
     use Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage;
     use Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler;
 
-    $pdo = new \PDO('mysql:dbname=testdb;host=127.0.0.1');
+    $pdo = new \PDO(...);
     $storage = new NativeSessionStorage(array(), new PdoSessionHandler($pdo));
     $session = new Session($storage);