docs: update language setting for Sphinx build

docs: update language setting for Sphinx build

Sphinx 5.0.0 no longer accepts None for language. Remove it and Sphinx
will use English as the default and no longer emit a warning.

For details see https://github.com/sphinx-doc/sphinx/issues/10474

Sphinx warnings cause our documentation build on GitHub Actions to fail.
Failures were observed for recent macOS builds which upgraded to Sphinx
5.0.0.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>
Link: https://lore.kernel.org/r/20220531214857.169864-1-vincent.fu@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>