fixed links in docs

doc/providers/form.rst

@@ -201,4 +201,4 @@ Traits
     $app->form($data);
 
 For more information, consult the `Symfony Forms documentation
-<http://symfony.com/doc/2.3/book/forms.html>`_.
+<http://symfony.com/doc/2.8/book/forms.html>`_.

doc/providers/security.rst

@@ -90,7 +90,7 @@ Usage
 
 The Symfony Security component is powerful. To learn more about it, read the
 `Symfony Security documentation
-<http://symfony.com/doc/2.3/book/security.html>`_.
+<http://symfony.com/doc/2.8/book/security.html>`_.
 
 .. tip::