nixos/stalwart-mail: fixed broken link

nixos/modules/services/mail/stalwart-mail.nix

···
       default = { };
       description = mdDoc ''
         Configuration options for the Stalwart email server.
-        See <https://stalw.art/docs/> for available options.
+        See <https://stalw.art/docs/category/configuration> for available options.
 
         By default, the module is configured to store everything locally.
       '';