#17257 change subtitle of top page

Todo:
* [x] change tag &lt;h2&gt; of file app/views/messages/index.php
* [x] check: http://localhost/fuel_intro/fuel/public/messages
* [x] check by nemoto: http://localhost/fuel_intro/fuel/public/messages

I think , the comment is not enough.
I guess, "Create new branch for #17257:  change sub title of top page" is better, how do you think?