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

docs/programmers/api-functions/api-register-default-plugin-handler.md

@@ -25,7 +25,7 @@ plugin types.
 
 
     <?php
-
+    use Smarty\Smarty;
     $smarty = new Smarty();
     $smarty->registerDefaultPluginHandler('my_plugin_handler');