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 <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>

Author: johnkeeping

Documentation/conf.py

@@ -135,7 +135,7 @@
 # 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 ----------------------------------------------