fixed doc

doc/changelog.rst

@@ -12,11 +12,11 @@ Changelog
 
     After::
 
-        $app['twig'] = $app->extend('twig', function($twig, $app) {
+        $app['twig'] = $app->share($app->extend('twig', function($twig, $app) {
             // do something
 
             return $twig;
-        });
+        }));
 
 * **2012-06-13**: Added a route ``before`` middleware
 

doc/providers/twig.rst

@@ -126,12 +126,12 @@ Customization
 You can configure the Twig environment before using it by extending the
 ``twig`` service::
 
-    $app['twig'] = $app->extend('twig', function($twig, $app) {
+    $app['twig'] = $app->share($app->extend('twig', function($twig, $app) {
         $twig->addGlobal('pi', 3.14);
         $twig->addFilter('levenshtein', new \Twig_Filter_Function('levenshtein'));
 
         return $twig;
-    });
+    }));
 
 For more information, check out the `Twig documentation
 <http://twig.sensiolabs.org>`_.