config(mdformat): Use older package

Newest one have problem with mkdocs version

Author: MartinNikov

flake.nix

@@ -150,7 +150,7 @@
               "*.md"
               "*.html"
             ];
-            mdformat.package = lib.mkDefault (pkgsUnstable.mdformat.withPlugins (p: [
+            mdformat.package = lib.mkDefault (pkgs.mdformat.withPlugins (p: [
               p.mdformat-admon
               p.mdformat-beautysh
               p.mdformat-footnote