docs/manual: FOO_SITE must not have a trailing slash

This has been the best practice since a long time, but it was not
clearly stated in the manual. Now it is mandatory, so mention it
explicitly.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>