docs: improve sample code

Author: kenjis

user_guide_src/source/libraries/files/012.php

@@ -1,6 +1,6 @@
 <?php
 
-$files->removeFile(APPPATH . 'Filters/DevelopToolbar');
+$files->removeFile(APPPATH . 'Filters/DevelopToolbar.php');
 
 $files->removePattern('#\.gitkeep#');
 $files->retainPattern('*.php');