nix-darwin: add modulesPath to specialArgs (#2375)

Same as 099cbcf13e, however now for
nix-darwin instead of NixOS.

Tested with `disabledModules = [ "programs/alacritty.nix" ];` and having an
error.

nix-darwin/default.nix

@@ -13,6 +13,7 @@ let
       lib = extendedLib;
       darwinConfig = config;
       osConfig = config;
+      modulesPath = ../modules;
     } // cfg.extraSpecialArgs;
     modules = [
       ({ name, ... }: {
@@ -62,7 +63,6 @@ in
       extraSpecialArgs = mkOption {
         type = types.attrs;
         default = { };
-        example = literalExample "{ modulesPath = ../modules; }";
         description = ''
           Extra <literal>specialArgs</literal> passed to Home Manager.
         '';