change nix formatter to nixfmt-tree

flake.nix

@@ -6,7 +6,7 @@
   };
 
   outputs =
-    inputs@{
+    {
       self,
       nixpkgs,
     }:
@@ -28,11 +28,11 @@
           }
         );
     in
-    rec {
+    {
       devShells = forAllSystems (
         { pkgs, system }:
         {
-          default = pkgs.mkShell rec {
+          default = pkgs.mkShell {
             packages = with pkgs; [
               (pkgs.symlinkJoin {
                 name = "rust-toolchain";
@@ -55,6 +55,6 @@
         }
       );
 
-      formatter = forAllSystems ({ pkgs, ... }: pkgs.nixfmt-rfc-style);
+      formatter = forAllSystems ({ pkgs, ... }: pkgs.nixfmt-tree);
     };
 }