fix defaultText for pkgs option using literalMD

Passing a non-literal string to `example` or `defaultText` causes the
docs to render the value using `lib.generators.toPretty`.

If you want to use markdown, use `literalMD`. If you want to pass a nix
expression, use `literalExpression`.

Author: MattSturgeon

flake-module.nix

@@ -24,7 +24,7 @@ in
             modules = (import ./.).submodule-modules ++ [{
               options.pkgs = lib.mkOption {
                 default = pkgs;
-                defaultText = "`pkgs` (module argument of `perSystem`)";
+                defaultText = lib.literalMD "`pkgs` (module argument of `perSystem`)";
               };
               options.flakeFormatter = lib.mkOption {
                 type = types.bool;