nixos/tests/phabricator: fix renaming warning

nixos/tests/phabricator.nix

···
     client =
       { config, pkgs, ... }:
       { imports = [ ./common/x11.nix ];
-        services.xserver.desktopManager.kde5.enable = true;
+        services.xserver.desktopManager.plasma5.enable = true;
       };
   };