Resolves: fdo#74820 Failed to recognized Heading 6-Export to MediaWiki

See http://www.mediawiki.org/wiki/Help:Formatting

Change-Id: I6cf50e957d3aef6bff6e86a031da175d5544e98c
Reviewed-on: https://gerrit.libreoffice.org/7998
Reviewed-by: Joren De Cuyper <jorendc@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>