Update irc channel for security sig meetings

This change updates the channel used for the openstack security
sig meetings to the main openstack-security irc channel.

Change-Id: I9a38afec6cb69e15acac792482c2950fdbae495f

meetings/openstack-security-sig-meeting.yaml

@@ -3,7 +3,7 @@ meeting_id: security
 schedule:
   - time:       '1500'
     day:        Thursday
-    irc:        openstack-meeting
+    irc:        openstack-security
     frequency:  first-thursday
     start_date: 20180308
 chair: Gage Hugo (gagehugo)