Update etc example to not use a real config file

[grahamc]

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

A user noticed the example for hosts, took the mode permissions literally, and ended up with surprising behavior on their system. Updating the documentation to not reference a real config file which might have real permissions requirements.

[mention-bot]

By analyzing the blame information on this pull request, we identified @edolstra, @thoughtpolice and @nbp to be potential reviewers

[grahamc]

Ref: https://github.com/NotaseCretagen/nixosian/blob/master/configuration.nix#L708-L725

[NotaseCretagen]

That link will change with file contents, using this fixed one for future reference: https://github.com/NotaseCretagen/nixosian/blob/cb5fefc97671a27e23852de1c0c877e4133e0301/configuration.nix#L708-L725

[NotaseCretagen]

Is the file in this commit (nixos/modules/system/etc/etc.nix) what's being used here in the manual?
https://nixos.org/releases/nixos/unstable/nixos-16.09pre79453.32b7b00/manual/options.html#opt-environment.etc

EDIT: nevermind, I guess so! (even though I don't know how it's being used like that)
https://github.com/NixOS/nixpkgs/search?utf8=%E2%9C%93&q=%22%2Fnix%2Fstore%2F...%2Fetc%2Fdir%2Ffile.conf.example%22&type=Code

[joachifm]

Makes sense