Docs: fix table problems in ras.rst

Two table problems caused the PDF build to fail:

 - Evidently multirow cells are not appreciated in table headers,
   so remove such from the "CS Rows" table.

 - The logging message structure table was incorrectly formatted,
   with two "+" instead of "|".  The HTML build is forgiving of such
   things, but PDF is not.

Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>