When iWeb publishes a website it will include the sitename in the URL. That is how iWeb works. You have two solutions. You can either turn on domain masking which I DO NOT RECOMMEND as it will cause other issues or you can publish using a third party utility.
Domain masking makes it so that no matter what page you are on your domain name, http://www.example.com, is the only thing that shows up in your browser. This may look cleaner but it prevents users from bookmarking your website and will prevent your website from being fully added to search engines. You would have to set this up with your web hosting service.
The other option will require that you publish your website to a folder and then use a third party utility to publish your site. You can use an FTP client such as cyberduck. We also developed a tool that will help you get this short URL. The step by step video tutorial can be seen here;
http://www.ragesw.com/blog/2010/08/24/how-to-create-visitor-friendly-short-urls- with-iweb/
Hope this helps.