docs: improve sample code

To make it easier to understand.

Author: kenjis

user_guide_src/source/database/query_builder/103.php

@@ -1,11 +1,10 @@
 <?php
 
-$union   = $db->table('users')->select('id, name');
-$builder = $db->table('users')->select('id, name');
-
-$builder->union($union)->limit(10)->get();
+$builder = $db->table('users')->select('id, name')->limit(10);
+$union   = $db->table('groups')->select('id, name');
+$builder->union($union)->get();
 /*
  * Produces:
  * SELECT * FROM (SELECT `id`, `name` FROM `users` LIMIT 10) uwrp0
- * UNION SELECT * FROM (SELECT `id`, `name` FROM `users`) uwrp1
+ * UNION SELECT * FROM (SELECT `id`, `name` FROM `groups`) uwrp1
  */