docs: Correct fallback user config path
docs: Correct fallback user config path

This is in line with XDG Base Directory Specification, where ~/.config is supposed to be used when XDG_CONFIG_HOME is unset.

It also better matches the reality, where ~/.config/nix.conf does not seem to be used.
NixOS/nix,rycee/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix