docs: networking: nfc: fix code block syntax

Silence this warning:

Documentation/networking/nfc.rst:113: WARNING: Definition list ends without
a blank line; unexpected unindent.

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