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

Documentation/conf.py

@@ -135,7 +135,7 @@ pygments_style = 'sphinx'
 # If true, `todo` and `todoList` produce output, else they produce nothing.
 todo_include_todos = False
 
-primary_domain = 'C'
+primary_domain = 'c'
 highlight_language = 'none'
 
 # -- Options for HTML output ----------------------------------------------