doc: updating slack invitation link

As mentioned in a github issue comment
https://github.com/cuelang/cue/issues/63#issuecomment-525383056
the slack invitation link is dead and needed to be updated.

Issue #63

Change-Id: Idd882123a422f758cc733366553342f63d9c5775
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3020
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/README.md b/README.md
index 4b8d79c..b885dc7 100644
--- a/README.md
+++ b/README.md
@@ -149,7 +149,7 @@
 You can get in touch with the cuelang community in the following ways:
 
 - Chat with us on our
-  [Slack workspace](https://join.slack.com/t/cuelang/shared_invite/enQtNjk1NzU1MzMwMjI2LWQ0ZDA2OTRjYTQ0MGUyZDBiYWIzZjVhZGI1YWVlNTVmMWU3MTFhY2MxZmRmMjBiMTA5MGQ1YzUyMmMzN2IxYzc).
+  [Slack workspace](https://join.slack.com/t/cuelang/shared_invite/enQtNzQwODc3NzYzNTA0LTAxNWQwZGU2YWFiOWFiOWQ4MjVjNGQ2ZTNlMmIxODc4MDVjMDg5YmIyOTMyMjQ2MTkzMTU5ZjA1OGE0OGE1NmE).
 
 
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