nixos/dendrite: fix broken example conf link (#342056)

      default = { };

      description = ''

        Configuration for dendrite, see:

        <https://github.com/matrix-org/dendrite/blob/master/dendrite-config.yaml>

        <https://github.com/matrix-org/dendrite/blob/main/dendrite-sample.yaml>

        for available options with which to populate settings.

      '';

    };