doc: kerneldoc.py: fix indentation

kerneldoc.py is mostly indented with 4 spaces (like PEP8 suggests);
replace the last remaining tabs for consistency.

No functional change.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240215134828.1277109-6-vegard.nossum@oracle.com