Update readme-slack.md
Update readme-slack.md

Missing the word 'how'.
ouadie-lahdioui/botkit,healthchatnow/hcn,EliKrumholz/botkit,howdyai/botkit,acrollet/botkit,dhulser/slack-reporting-bot,DJ1K1/botkit,DJ1K1/botkit,howdyai/botkit,howdyai/botkit,samuelsan/YelpBot,howdyai/botkit