doc: Fix refentry id in flatpak-build-sign.xml
doc: Fix refentry id in flatpak-build-sign.xml

The id value in flatpak-build-sign.xml is "flatpak-build-export", which
causes the generated HTML to link to that command instead. Fix it.

handsome-feng/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak