README.md: update ClusterAPI meeting agenda link

The link to ClusterAPI's weekly meetings' agenda / minutes doc points to a document that's inaccessible.
This change updates that link to https://cluster-api.sigs.k8s.io/agenda which points to the correct agenda doc.

Author: t-lo

README.md

@@ -51,7 +51,7 @@ See also our [contributor guide](CONTRIBUTING.md) and the Kubernetes [community
 Participation in the Kubernetes community is governed by the [Kubernetes Code of Conduct](code-of-conduct.md).
 
 [community page]: https://kubernetes.io/community
-[notes]: https://docs.google.com/document/d/1ushaVqAKYnZ2VN_aa3GyKlS4kEd6bSug13xaXOakAQI
+[notes]: https://cluster-api.sigs.k8s.io/agenda
 [recordings]: https://www.youtube.com/playlist?list=PL69nYSiGNLP29D0nYgAGWt1ZFqS9Z7lw4
 [zoomMeeting]: https://zoom.us/j/861487554
 [implementerNotes]: https://docs.google.com/document/d/1IZ2-AZhe4r3CYiJuttyciS7bGZTTx4iMppcA8_Pr3xE/edit