docs: update slack invite link

lib/instrumentation/superagent/CONTRIBUTING.md

@@ -43,8 +43,7 @@ This?”](https://infrequently.org/2008/06/why-do-i-need-to-sign-this/).
 We host a public Slack with a dedicated channel for contributors and
 maintainers of open source projects hosted by New Relic. If you are
 contributing to this project, you're welcome to request access to the
-\#oss-contributors channel in the newrelicusers.slack.com workspace. To request
-access, see https://newrelicusers-signup.herokuapp.com/.
+\#oss-contributors channel in the newrelicusers.slack.com workspace. To request access, please use this [link](https://join.slack.com/t/newrelicusers/shared_invite/zt-1ayj69rzm-~go~Eo1whIQGYnu3qi15ng).
 
 ## PR Guidelines