simple: update example

modules/examples/simple.nix

@@ -1,6 +1,6 @@
-{ config, lib, pkgs, ... }:
-{
+{ config, pkgs, ... }:
 
+{
   # List packages installed in system profile. To search by name, run:
   # $ nix-env -qaP | grep wget
   environment.systemPackages =
@@ -10,9 +10,6 @@
   # Create /etc/bashrc that loads the nix-darwin environment.
   programs.bash.enable = true;
 
-  # Recreate /run/current-system symlink after boot.
-  services.activate-system.enable = true;
-
   # Used for backwards compatibility, please read the changelog before changing.
   # $ darwin-rebuild changelog
   system.stateVersion = 2;
@@ -20,4 +17,5 @@
   # You should generally set this to the total number of logical cores in your system.
   # $ sysctl -n hw.ncpu
   nix.maxJobs = 1;
+  nix.buildCores = 1;
 }