[ZF-11306] Docs Zend_Auth

- Fix undefined variable in example documentation.

git-svn-id: http://framework.zend.com/svn/framework/standard/trunk@23866 44c647ce-9c0f-0410-b52a-842ac1e357ba

documentation/manual/en/tutorials/multiuser-authentication.xml

@@ -143,6 +143,7 @@ class AuthController extends Zend_Controller_Action
             $adapter->setIdentity($loginForm->getValue('username'));
             $adapter->setCredential($loginForm->getValue('password'));
 
+            $auth   = Zend_Auth::getInstance(); 
             $result = $auth->authenticate($adapter);
 
             if ($result->isValid()) {