Desktop file fixes

This commit fixes two errors reported by `desktop-file-validate`:

    $ desktop-file-validate openambit.desktop 
    openambit.desktop: error: value "HEAD" for key "Version" in group "Desktop Entry" is not a known version
    openambit.desktop: error: value "Utility;Education;Sports" for string list key "Categories" in group "Desktop Entry" does not have a semicolon (';') as trailing character