scripts: sphinx-pre-install: change recommendation text if venv exists

If one is running a Sphinx version older than what's recommended,
but there's already a newer working virtual env, change the
text, as it is just a matter of switching to the new venv, instead
of creating a new one from scratch.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/bcf79d0399a1c3444ca938dcdce599c3273980ab.1587478901.git.mchehab+huawei@kernel.org


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