docs: fix db group name in sample code

It is a property of the Config class. It is not likely PascalCase.

Author: kenjis

user_guide_src/source/concepts/factories/004.php

@@ -1,4 +1,4 @@
 <?php
 
-$conn  = db_connect('AuthDatabase');
+$conn  = db_connect('auth');
 $users = Factories::models('UserModel', [], $conn);