Update code examples to use Smarty\Smarty and autoload instead of require_once calls and new Smarty

docs/programmers/api-variables/variable-default-config-handler-func.md

@@ -12,7 +12,7 @@ resource.
 
 
     <?php
-
+    use Smarty\Smarty;
     $smarty = new Smarty();
     $smarty->default_config_handler_func = 'my_default_config_handler_func';