Correct documentation for ConfbridgeList AMI action

The documentation for ConfbridgeList states that the Conference field is
optional. That's not really the case: if you fail to provide a Conference
number, the command will kick back an error.

(closes issue AST-1090)
Reported by: John Bigelow


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@380028 f38db490-d61c-443f-a65b-d21fe96a405b