Unbreak servant-docs

[magthe]

Motivation for this change

It's a useful package and it builds as is without any overrides.

Things done

Built locally using nix-build -A haskellPackages.servant-docs --arg config '{ allowBroken = true; }' without issues.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[sternenseemann]

Thank you! I've merged your commit as cb97f4a (just reworded the message to fit the guideline). It'll be merged with #119652.