Documentation/sphinx: fix primary_domain configuration

With Sphinx 1.5.3 I get the warning:

	WARNING: primary_domain 'C' not found, ignored.

It seems that domain names in Sphinx are case-sensitive and for the C
domain the name must be lower case.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>