[whiteboard] Update manifest.ini

whiteboard/manifest.ini

 [info]
 name: Whiteboard
 short_name: whiteboard
-version: 1.2.5
+version: 1.2.6
 description: Shows a whiteboard in chat. python-pygoocanvas is required.
 authors = Yann Leboulanger <asterix@lagaule.org>
 homepage = https://dev.gajim.org/gajim/gajim-plugins/wikis/WhiteboardPlugin