docs: replace $this->db with $db

Other sample codes use $db.

Author: kenjis

user_guide_src/source/database/query_builder/103.php

@@ -1,7 +1,7 @@
 <?php
 
-$union   = $this->db->table('users')->select('id', 'name');
-$builder = $this->db->table('users')->select('id', 'name');
+$union   = $db->table('users')->select('id', 'name');
+$builder = $db->table('users')->select('id', 'name');
 
 $builder->union($union)->limit(10)->get();
 /*

user_guide_src/source/database/query_builder/104.php

@@ -1,9 +1,9 @@
 <?php
 
-$union   = $this->db->table('users')->select('id', 'name')->orderBy('id', 'DESC')->limit(5);
-$builder = $this->db->table('users')->select('id', 'name')->orderBy('id', 'ASC')->limit(5)->union($union);
+$union   = $db->table('users')->select('id', 'name')->orderBy('id', 'DESC')->limit(5);
+$builder = $db->table('users')->select('id', 'name')->orderBy('id', 'ASC')->limit(5)->union($union);
 
-$this->db->newQuery()->fromSubquery($builder, 'q')->orderBy('id', 'DESC')->get();
+$db->newQuery()->fromSubquery($builder, 'q')->orderBy('id', 'DESC')->get();
 /*
  * Produces:
  * SELECT * FROM (