blog/nix-flakes-1-2022-02-21 flake.nix from template is different

The way default package is declared in the template you suggest using (https://github.com/NixOS/templates/blob/master/go-hello/flake.nix) is different from what the current version of the blog says it should be. This breaks the instructions for some sections where the default package is referenced by name unless the reader spots the difference in flake.nix initially.