Modify README Instruction for Flake

[rickypang0219]

This PR modifies the instruction using flake to install Fenix for Rust Overlay. I am a Mac user and I use nix-flake and Nix-darwin to config my MacBook. I encounter an attribute error by following the installation instruction of using Flake.

error: attribute 'complete' missing
       at /nix/store/p2ppjg1xw47ilb00ypk3kfng06gn63ad-source/nix-darwin/flake.nix:77:18:
           76|             environment.systemPackages = with pkgs; [
           77|                 (fenix.complete.withComponents [
             |                  ^
           78|                   "cargo"

To solve this problem, I modify part of the Flake instruction fenix.complete.withComponents to fenix.packages.${system}.complete.withComponents then it works on my M1 MacBook Pro. I am not sure is this a proper way to install Fenix with Nix Flakes and Nix-Darwin in Macs.