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>

Documentation/networking/nfc.txt

@@ -103,14 +103,14 @@ it's closed.
 LOW-LEVEL DATA EXCHANGE:
 
 The userspace must use PF_NFC sockets to perform any data communication with
-targets. All NFC sockets use AF_NFC:
-
-struct sockaddr_nfc {
-       sa_family_t sa_family;
-       __u32 dev_idx;
-       __u32 target_idx;
-       __u32 nfc_protocol;
-};
+targets. All NFC sockets use AF_NFC::
+
+        struct sockaddr_nfc {
+               sa_family_t sa_family;
+               __u32 dev_idx;
+               __u32 target_idx;
+               __u32 nfc_protocol;
+        };
 
 To establish a connection with one target, the user must create an
 NFC_SOCKPROTO_RAW socket and call the 'connect' syscall with the sockaddr_nfc