docs: Add colon clearing sphinx warning

Sphinx emits various warnings all caused by a missing colon before code
block:

	WARNING: Block quote ends without a blank line; unexpected unindent.
	ERROR: Unexpected indentation.
	WARNING: Block quote ends without a blank line; unexpected unindent.

Add the colon, clearing sphinx warnings.

Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>