nixos: fix renaming warning in KDE closure

nixos/release.nix

···
     kde = makeClosure ({ pkgs, ... }:
       { services.xserver.enable = true;
         services.xserver.displayManager.sddm.enable = true;
-        services.xserver.desktopManager.kde5.enable = true;
+        services.xserver.desktopManager.plasma5.enable = true;
       });
 
     xfce = makeClosure ({ pkgs, ... }: