nixos/documentation: Correct use of lib.optional

[JohnAZoidberg]

lib.optional returns a singleton or an empty list. Therefore the
argument does not need to (or rather 'must not') be wrapped in a list.
An alternative patch could have used lib.optionals but seems like no
more elements are going to be added to the optional list.

Motivation for this change

Doesn't build because the types don't match.
Now it does.

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
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• 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.

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