docs: Add missing colon

sphinx emits warning

       WARNING: Inline emphasis start-string without end-string.

This is caused by a missing colon.

Add missing colon, clearing shpinx build warning.

Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>