Simple!
You typed "joomla" in the little box when you installed it, which automatically created a "directory" that it was installed to.
If you are going to use Joomla to create your website for your domain, then you need to install Joomla to the
public_html/ (root) directory, so when you go to your domain name, it appears first....right?
So, to correct the problem, you will need to uninstall Joomla, and then re-install it --- but this time do not fill in any boxes or 'specifiy' a destination: it will install itself directly on your root directory.
No big, but it pays to read up first....for solutions and help, you should become familiar with the Official Joomla Support Forum!