ut2004demo: fix download URL

http://treefort.icculus.org/ut2004/UT2004-LNX-Demo3334.run.gz is gone.

pkgs/games/ut2004/demo.nix

···
   version = "3334";
 
   src = fetchurl {
-    url = "http://treefort.icculus.org/ut2004/UT2004-LNX-Demo${version}.run.gz";
+    url = "http://vlaai.snt.utwente.nl/pub/games/UT2004/demo/UT2004-LNX-Demo${version}.run.gz";
     sha256 = "0d5f84qz8l1rg16yzx2k4ikr46n9iwj68na1bqi87wrww7ck6jh7";
   };