app_meetme: Fix manager event documentation for several events.

The MeetmeJoin, MeetmeLeave, MeetmeEnd, MeetmeMute, MeetmeTalking, and
MeetmeTalkRequest AMI events were documented with sending out a Usernum
header when the User header was actually output.

* Change the online documentation to match reality.

ASTERISK-27873
ASTERISK-25261

Change-Id: I437bc70618d07c183c9624b7069c2fcae7f17a39

apps/app_meetme.c

@@ -562,7 +562,7 @@
 				<parameter name="Meetme">
 					<para>The identifier for the MeetMe conference.</para>
 				</parameter>
-				<parameter name="Usernum">
+				<parameter name="User">
 					<para>The identifier of the MeetMe user who joined.</para>
 				</parameter>
 				<channel_snapshot/>