Fix slack url

Change the slack url to the self-signup variant.

This fixes #272.

README.md

@@ -36,7 +36,7 @@ Join the Community
 * Contact the [official mailing list](https://groups.google.com/forum/#!forum/falco) for support and to talk with other users.
 * Follow us on [Twitter](https://twitter.com/sysdig) for general falco and sysdig news.
 * This is our [blog](https://sysdig.com/blog/), where you can find the latest [falco](https://sysdig.com/blog/tag/falco/) posts.
-* Join our [Public Slack](https://sysdig.slack.com) channel for sysdig and falco announcements and discussions.
+* Join our [Public Slack](https://slack.sysdig.com) channel for sysdig and falco announcements and discussions.
 
 License Terms
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