docs/manual: fix packages having a subdirectory

efl and matchbox no longer have a subdirectory containing multiple
packages, so they are no longer good examples of that. Mention qt5 and
gstreamer instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>