Fix missing doc tags found while fixing ASTERISK-18689

Add missing <variable></variable> tags in app_dial documentation.
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Merged revisions 348992 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@348993 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_dial.c

@@ -476,7 +476,7 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 			<para>If the <variable>OUTBOUND_GROUP</variable> variable is set, all peer channels created by this
 			application will be put into that group (as in Set(GROUP()=...).
 			If the <variable>OUTBOUND_GROUP_ONCE</variable> variable is set, all peer channels created by this
-			application will be put into that group (as in Set(GROUP()=...). Unlike OUTBOUND_GROUP,
+			application will be put into that group (as in Set(GROUP()=...). Unlike <variable>OUTBOUND_GROUP</variable>,
 			however, the variable will be unset after use.</para>
 
 			<para>This application sets the following channel variables:</para>