Fix Kosovo alpha-3 code

[opeitscope]

According to https://en.wikipedia.org/wiki/XK_(user_assigned_code)#:~:text=XK%20is%20an%20ISO%203166,used%20in%20the%20Unicode%20standard the ISO 3166-1 alpha-3 code for Kosovo is XKX

[mzdun]

Please note PR #356 claims it's XKK, basing this on Wikipedia, but Wiki XK page as of today (Jan 2nd 2025) states, XKK is used in Unicode and XKX is used as ISO alpha3 equivalent. This library seems to be ISO centric, so XKX should be used, in my opinion and the code needs to ba somehow guarded from future attempts at changing it back to XKK.