docs: load_config.py: ensure subdirs end with "/"

The logic with seeks for a subdir passed via SPHINXDIRS is
incomplete: if one uses something like:

	make SPHINXDIRS=arm pdfdocs

It will find both "arm" and "arm64" directories. Worse than
that, it will convert "arm64/index" to "4/index".

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>