lightdm: add description

pkgs/applications/display-managers/lightdm/default.nix

···
 
   meta = {
     homepage = https://github.com/CanonicalLtd/lightdm;
+    description = "A cross-desktop display manager.";
     platforms = platforms.linux;
     license = licenses.gpl3;
     maintainers = with maintainers; [ ocharles wkennington worldofpeace ];