klettres: update homepage

pkgs/applications/kde/klettres.nix

···
 mkDerivation {
   pname = "klettres";
   meta = with lib; {
-    homepage = "https://kde.org/applications/en/utilities/org.kde.klettres";
+    homepage = "https://invent.kde.org/education/klettres";
     description = "An application specially designed to help the user to learn an alphabet";
     maintainers = with maintainers; [ freezeboy ];
     license = licenses.gpl2Plus;