Update rasa_core/channels/slack.py
Update rasa_core/channels/slack.py

Co-Authored-By: mark-collins-voxsio <ae7ab197e046d82fdf6befbdcf55245846148269@users.noreply.github.com>
RasaHQ/rasa_core,RasaHQ/rasa_nlu,RasaHQ/rasa_nlu,RasaHQ/rasa_nlu,RasaHQ/rasa_core,RasaHQ/rasa_core