Fix categories in .desktop file to make Rhythmbox show up in gnome-software

gnome-software uses the desktop file categories to sort the applications
it shows.

This commit fixes the categories according to the freedesktop spec,
so that Rhythmbox will appear in the correct category.

For more information:
http://standards.freedesktop.org/menu-spec/latest/apa.html
http://standards.freedesktop.org/menu-spec/latest/apas02.html