Documentation/sphinx: rename kernel-doc.py to kerneldoc.py

Python module names should not have hyphens per [PEP 8]. Drop the hyphen
from kernel-doc.py. The extension directive remains unchanged.

[PEP 8] https://www.python.org/dev/peps/pep-0008/#package-and-module-names



Reported-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>