Update nix-darwin URL to new organization

Author: ntsk

nix/flake.nix

@@ -8,7 +8,7 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     nix-darwin = {
-      url = "github:LnL7/nix-darwin";
+      url = "github:nix-darwin/nix-darwin";
       inputs.nixpkgs.follows = "nixpkgs";
     };
   };