[DOC] #123 Provide correct link to "XMPP server" page

Change-Id: I9284e92e241fe5d5dcb4cb8379eb20d110c972cf
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1294
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Tested-by: Jenkins CI