fix nix aspell installation instructions
fix nix aspell installation instructions

The original instruction made nix install the language packages in different directories than the `aspell` package.

Use solution provided here https://github.com/hlissner/doom-emacs/issues/4138#issuecomment-717266771


closes https://github.com/hlissner/doom-emacs/issues/4138
hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d