docs: fix mailing list URL

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/index.md b/docs/index.md
index 3c4d1a3..e0c11db 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -65,7 +65,7 @@
 
 Developers welcome!
 
-* [Our developer mailing list](http://lists.runtime.co/mailman/listinfo/dev-mcuboot_lists.runtime.co)
+* [Our developer mailing list](https://groups.io/g/MCUBoot)
 * [Our Slack channel](https://mcuboot.slack.com/)<br />
   Get your invite [here!](https://join.slack.com/t/mcuboot/shared_invite/MjE2NDcwMTQ2MTYyLTE1MDA4MTIzNTAtYzgyZTU0NjFkMg)
 * [Our IRC channel](http://irc.freenode.net), channel #mcuboot ([IRC