Update Slack URL

Use URL that actually lets you join w/o an invitation (hopefully this won't attract spam)

docs/participate.md

@@ -8,7 +8,7 @@ The OBO Foundry is currently a volunteer-run organization. There are many ways t
 - Submit a bug report, question, or feature request to the [OBO Foundry issue tracker](https://github.com/OBOFoundry/OBOFoundry.github.io/issues)
 - Join the [OBO-Discuss mailing list](https://groups.google.com/forum/#!forum/obo-discuss)
 - Join the [OBO-Tools mailing list](https://groups.google.com/forum/#!members/obo-tools)
-- Participate in the [OBO Community Slack channel](https://obo-communitygroup.slack.com/archives/C01DP18L5GW)
+- Participate in the [OBO Community Slack channel](https://join.slack.com/t/obo-communitygroup/shared_invite/zt-1oq48ttk7-kKo0i6TwntYtAq~Jcjjg4g)
 - [Submit your ontology](/faq/how-do-i-register-my-ontology.html) to be considered for inclusion in the OBO Foundry
 - [Create a pull request to improve this website](/faq//how-do-i-modify-website.html)
 - Nominate yourself to [join the OBO Foundry Operations Committee](/docs/NewOBOFC.html)