message: Fix heading for NixOS/Nixpkgs
  The level of heading were one to high.
diff: @@ -167,7 +167,7 @@ something is still found to be missing, please open an issue.
  sudo eopkg install fontconfig-devel
  ```
  -### NixOS/Nixpkgs
  +#### NixOS/Nixpkgs
  The following command can be used to get a shell with all development
  dependencies on [NixOS](https://nixos.org).
  