Update RTM to Socket in BotManFactory method name

[michaelmcandrew]

I think you might have forgotten to update the method name in https://github.com/botman/docs/blob/master/driver-slack.md from BotManFactory::createForRTM to BotManFactory::createForSocket. Maybe the file and class names in https://github.com/botman/driver-slack/tree/master/src also need updating?