Update install_ut2k4_key.sh

The old URL gives a 404.

functions/install_ut2k4_key.sh

@@ -10,10 +10,10 @@ echo "================================="
 sleep 1
 echo "To get your server listed on the Master Server list"
 echo "you must get a free CD key. Get a key here:"
-echo "http://www.unrealtournament.com/ut2004server/cdkey.php"
+echo "https://forums.unrealtournament.com/utserver/cdkey.php?2004"
 echo ""
 echo "Once you have the key enter it below"
 echo -n "KEY: "
 read CODE
 echo ""\""CDKey"\""="\""${CODE}"\""" > "${systemdir}/cdkey"
-echo ""
+echo ""