docs: add namespace to sample class

Author: kenjis

user_guide_src/source/incoming/routing/020.php

@@ -1,7 +1,9 @@
 <?php
 
+use App\Libraries\RSSFeeder;
+
 $routes->get('feed', static function () {
-    $rss = new \RSSFeeder();
+    $rss = new RSSFeeder();
 
     return $rss->feed('general');
 });