doc/Makefile: do not preserve ownership when installing docs

Remove the '-a' (archive) flag so that rsync does not preserve
the original owner when installing the documentation.
The installed files will now be owned by root when using "sudo",
rather than the original user.

Closes #375
Reported-by: Ｖ字龍(Vdragon) <Vdragon.Taiwan@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>