Update slack join link (#4429)

Author: jorgensd

README.md

@@ -14,7 +14,7 @@ DOLFIN and is actively developed.
 For questions about using DOLFINx, visit the [FEniCS
 Discourse](https://fenicsproject.discourse.group/) page or use the
 [FEniCS Slack channel](https://fenicsproject.slack.com/) (use
-[this](https://join.slack.com/t/fenicsproject/shared_invite/zt-1lraknsp1-6_3Js5kueDIyWgF192d3nA)
+[this](https://join.slack.com/t/fenicsproject/shared_invite/zt-2wa76d0xw-ecfxcPj25K8pc7q7~1AK8g)
 link to sign up to the Slack channel).
 
 ## Documentation