man: fix prefixes for weston.ini(5)

Replace a few hardcoded paths with the substitutes

https://gitlab.freedesktop.org/wayland/weston/issues/105



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>