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 <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/conf.py

···
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['kernel-doc', 'rstFlatTable', 'kernel_include', 'cdomain']
+extensions = ['kerneldoc', 'rstFlatTable', 'kernel_include', 'cdomain']
 
 # The name of the math extension changed on Sphinx 1.4
 if minor > 3: