Fix signature of spawn method

[Naktibalda]

According to documentation at https://github.com/howdyai/botkit/blob/master/packages/docs/core.md#sending-alerts-and-scheduled-messages, config parameter of spawn() is optional.

However I got an error when compiling my bot:

src/actions.ts:117:42 - error TS2554: Expected 1 arguments, but got 0.
117                     const newBot = await bot.controller.spawn();
                                             ~~~~~~~~~~~~~~~~~~~~~~
  node_modules/botkit/lib/core.d.ts:502:11
    502     spawn(config: any): Promise<BotWorker>;
                  ~~~~~~~~~~~
    An argument for 'config' was not provided.