docs: fix minimal sphinx version in conf.py

according to what Documentation/doc-guide/sphinx.rst says::

  The ReST markups currently used by the Documentation/ files
  are meant to be built with ``Sphinx`` version 1.3 or upper.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>