docs: networking: nfc: change block diagram to sphinx syntax

Change the block diagram to match the sphinx syntax. This will make it
possible to switch this file to rst in the future.
Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

Documentation/networking/nfc.txt

@@ -26,6 +26,7 @@ The control operations are available to userspace via generic netlink.
 The low-level data exchange interface is provided by the new socket family
 PF_NFC. The NFC_SOCKPROTO_RAW performs raw communication with NFC targets.
 
+.. code-block:: none
 
         +--------------------------------------+
         |              USER SPACE              |