Update slack invite link (#1069)

<!--
⚠️ If you do not respect this template, your pull request will be
closed.
⚠️ Your pull request title should be short detailed and understandable
for all.
⚠️ Also, please add it in the CHANGELOG file under Unreleased section.
⚠️ If your pull request fixes an open issue, please link to the issue.

✅ I have added the tests to cover my changes.
✅ I have updated the documentation accordingly.
✅ I have read the CONTRIBUTING document.
-->

### Summary

Related to Qiskit/qiskit.org#2959

### Details and comments

README.md

@@ -21,7 +21,7 @@ uphold this code.
 
 We use [GitHub issues](https://github.com/Qiskit/qiskit-experiments/issues) for
 tracking requests and bugs. Please
-[join the Qiskit Slack community](https://ibm.co/joinqiskitslack)
+[join the Qiskit Slack community](https://qisk.it/join-slack)
 and use the [#experiments](https://qiskit.slack.com/archives/CGZDF48EN) channel for discussion and
 simple questions.
 For questions that are more suited for a forum we use the Qiskit tag in