[ExpressionLanguage] Add missing argument in example code

Author: kevin-verschaeve

components/expression_language/extending.rst

@@ -34,7 +34,7 @@ This method has 3 arguments:
     use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
 
     $language = new ExpressionLanguage();
-    $language->register('lowercase', function ($str) {
+    $language->register('lowercase', function ($arguments, $str) {
         return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
     }, function ($arguments, $str) {
         if (!is_string($str)) {