scripts/kernel-doc: fix parser for apostrophes

On ReST, adding a text like ``literal`` is valid. However,
the kernel-doc script won't handle it fine.

We really need this feature, in order to escape things like
%ph, with is found on some C files.

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