Fix typo in `src/nix/shell.md`
Fix typo in `src/nix/shell.md`

Changed `If not command` to `If no command`, also specified that the
default shell launched in `nix shell` can be specified with `$SHELL`.

NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix