nixos/cups-pdf: fix settings type (#438619)

nixos/modules/services/printing/cups-pdf.nix

···
   instanceSettings = name: {
     freeformType =
       with lib.types;
-      nullOr (oneOf [
-        int
-        str
-        path
-        package
-      ]);
+      attrsOf (
+        nullOr (oneOf [
+          int
+          str
+          path
+          package
+        ])
+      );
     # override defaults:
     # inject instance name into paths,
     # also avoid conflicts between user names and special dirs