flake.nix: update nix-darwin github org name

Congrats on making this a shared org!

Author: mweinelt

flake.lock

@@ -44,7 +44,7 @@
     };
 
     darwin = {
-      url = "github:LnL7/nix-darwin/nix-darwin-25.11";
+      url = "github:nix-darwin/nix-darwin/nix-darwin-25.11";
       inputs.nixpkgs.follows = "nixpkgs";
     };