docs: kdoc_output: fix handling of simple tables

docs: kdoc_output: fix handling of simple tables

Fix check for simple table delimiters.

ReST simple tables use "=" instead of "-". I ended testing it with
a table modified from a complex one, using "--- --- ---", instead
of searching for a real Kernel example.

Only noticed when adding an unit test and seek for an actual
example from kernel-doc markups.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <dea95337c05040f95e5a95ae41d69ddef0aaa8d6.1774256269.git.mchehab+huawei@kernel.org>