Update advanced.rst

Hello,

I just updated the class name 😉

doc/advanced.rst

@@ -189,7 +189,7 @@ If you want to access the current environment instance in your filter, set the
 ``needs_environment`` option to ``true``; Twig will pass the current
 environment as the first argument to the filter call::
 
-    $filter = new \Twig\TwigFilter('rot13', function (Twig_Environment $env, $string) {
+    $filter = new \Twig\TwigFilter('rot13', function (\Twig\Environment $env, $string) {
         // get the current charset for instance
         $charset = $env->getCharset();
 
@@ -208,7 +208,7 @@ the first argument to the filter call (or the second one if
         // ...
     }, ['needs_context' => true]);
 
-    $filter = new \Twig\TwigFilter('rot13', function (Twig_Environment $env, $context, $string) {
+    $filter = new \Twig\TwigFilter('rot13', function (\Twig\Environment $env, $context, $string) {
         // ...
     }, ['needs_context' => true, 'needs_environment' => true]);