maint: Remove mdDoc call

It has been removed upstream in Nixpkgs.
This fixes documentation rendering with the latest Nixpkgs.

nix/flake-module.nix

@@ -3,15 +3,14 @@ let
   inherit (flake-parts-lib)
     mkPerSystemOption;
   inherit (lib)
-    mdDoc
     mkOption
     types;
 in
 {
   options.perSystem = mkPerSystemOption ({ config, pkgs, lib, ... }:
     {
       options.process-compose = mkOption {
-        description = mdDoc ''
+        description = ''
           process-compose-flake: creates [process-compose](https://github.com/F1bonacc1/process-compose)
           executables from process-compose configurations written as Nix attribute sets.
         '';