docs: replace MySqli with MySQLi

Author: kenjis

user_guide_src/source/dbmgmt/forge/028.php

@@ -1,5 +1,5 @@
 <?php
 
-// MySqli Produces: ALTER TABLE `tablename` DROP PRIMARY KEY
+// MySQLi Produces: ALTER TABLE `tablename` DROP PRIMARY KEY
 // Others Produces: ALTER TABLE `tablename` DROP CONSTRAINT `pk_tablename`
 $forge->dropPrimaryKey('tablename');