nixos/networkmanager: cleanup, fix example rendering

  configFile = pkgs.writeText "NetworkManager.conf" (lib.concatStringsSep "\n" [

    (mkSection "main" {

      plugins = "keyfile";

      dhcp = cfg.dhcp;

      dns = cfg.dns;

      inherit (cfg) dhcp dns;

      # If resolvconf is disabled that means that resolv.conf is managed by some other module.

      rc-manager =

        if config.networking.resolvconf.enable then "resolvconf"

        example = literalExpression ''

          [ {

            source = pkgs.writeText "upHook" '''

              if [ "$2" != "up" ]; then

                logger "exit: event $2 != up"

                exit

              logger "Device $DEVICE_IFACE coming up"

            ''';

            type = "basic";

          } ]'';

          } ]

        '';

        description = lib.mdDoc ''

          A list of scripts which will be executed in response to network events.

        '';