Documentation: sphinx: Add missing comma to list of strings

In Python, like in C, when a comma is omitted in a list of strings, the
two strings around the missing comma are concatenated.

Cc: stable@vger.kernel.org  # v5.2 only
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>