message: MINOR: Fix broken slack link
diff: ### [harp.gl site](https://www.harp.gl/)
  -### [harp.gl Slack channel](https://heredev.slack.com/messages/harpgl/) Registration available [here](http://t.her.is/slack).
  +### [harp.gl Slack channel](https://heredev.slack.com/messages/harpgl/) Registration available [here](https://join.slack.com/t/heredev/shared_invite/zt-km24s2k3-lYwF3L96ml51Ue_SBoUM3A).
  ## Overview
  