root: update homepage

We try to promote the URL that is just `root.cern` nowadays.

pkgs/applications/science/misc/root/default.nix

···
   setupHook = ./setup-hook.sh;
 
   meta = with lib; {
-    homepage = "https://root.cern.ch/";
+    homepage = "https://root.cern/";
     description = "A data analysis framework";
     platforms = platforms.unix;
     maintainers = [ maintainers.guitargeek maintainers.veprbl ];