nixos/lxd: fix ui.enable desc

Otherwise it would show up as "Whether to enable Enables the (experimental) LXD UI. ."

Now is just "Whether to enable (experimental) LXD UI."

nixos/modules/virtualisation/lxd.nix

···
       };
 
       ui = {
-        enable = lib.mkEnableOption (lib.mdDoc ''
-          Enables the (experimental) LXD UI.
-        '');
+        enable = lib.mkEnableOption (lib.mdDoc "(experimental) LXD UI");
 
         package = lib.mkPackageOption pkgs.lxd-unwrapped "ui" { };
       };