trivial: set header for fwupdplugin documentation
trivial: set header for fwupdplugin documentation

hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd