Fix XML Document Validation Failure

Fix documentation error when validating the xml in trunk caused by r375150.
Moved the description end tag down to below the variablelist element end tag.

Found when compiling with --dev-mode-enabled.

(issue ASTERISK-20289)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375215 65c4cc65-6c06-0410-ace0-fbb531ad65f3