Fix doc render problem

This fixes:
```
RuntimeError: can't render html in the presence of docbook
```

modules/system/defaults/NSGlobalDomain.nix

@@ -228,7 +228,7 @@ in {
  system.defaults.NSGlobalDomain.NSWindowShouldDragOnGesture = mkOption {
  type = types.nullOr types.bool;
  default = null;
- description = ''
+ description = lib.mdDoc ''
  Whether to enable moving window by holding anywhere on it like on Linux. The default is false.
  '';
  };