docs: networking: nfc: change headlines to sphinx syntax

The headlines in this file do are not in the standard kernel docu-
mentation headline format. Change it, so this file can be switched to
rst in the future.

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>