scripts/kernel-doc: change the line number meta info

In order to make it more standard and ReST compatible,
change the meta-tag used with --enable-lineno from:

        #define LINENO

to
        .. LINENO

In practice, no	functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/40725032b5a4a33db740bf1de397523af958ff8a.1648290305.git.mchehab@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>