add TOUCH_MONITOR_PREF to the channel var docs

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@85683 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/tex/channelvariables.tex

@@ -831,6 +831,7 @@ ${QUEUESTATUS}            Status of the call, one of:
 ${RECORDED_FILE}        * Recorded file in record()
 ${TALK_DETECTED}        * Result from talkdetect()
 ${TOUCH_MONITOR}          The filename base to use with Touch Monitor (auto record)
+${TOUCH_MONITOR_PREF}   * The prefix for automonitor recording filenames.
 ${TOUCH_MONITOR_FORMAT}   The audio format to use with Touch Monitor (auto record)
 ${TOUCH_MONITOR_OUTPUT} * Recorded file from Touch Monitor (auto record)
 ${TXTCIDNAME}           * Result of application TXTCIDName