scripts: sphinx-pre-install: check for PDF min version later on

Better to add the PDF note late for venv recommendation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/8e117aabe6dfa1b1ec92dccd20e801393c977667.1656756450.git.mchehab@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>