Fixed typo in "Controllers in classes" paragraph

doc/usage.rst

@@ -453,7 +453,7 @@ If you don't want to use anonymous functions, you can also define your
 controllers as methods. By using the ``ControllerClass::methodName`` syntax,
 you can tell Silex to lazily create the controller object for you::
 
-    $app->get('/', 'Igorw\Foo::bar');
+    $app->get('/', 'Igorw\\Foo::bar');
 
     use Silex\Application;
     use Symfony\Component\HttpFoundation\Request;