Hi,
Keep this in mind. This is valid for all Joomla Installation.. Not just for installing the SportMart.
YOU SHOULD NEVER EVER ENTER ANYTHING to live site URL field during the installation, Joomla is clever enough to figure out what your website URL is when you first visit your website after the installation... (it is being asked in the official Joomla installment as well, under the "advanced" options)..
Entering the site live URL manually is for very very very rare situations. If you insist on doing it, then you must enter the correct URL, in the form of
www.yourdomain.com (with the http:// in the beginning and no slush at the end)...
Since this is a thumb-rule for any Joomla installment, in general, it isn't entirely necessary to put it into SportMart's tutorial.
Regards.