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

@@ -26,30 +26,31 @@ 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.
 
-
-             +--------------------------------------+
-             |              USER SPACE              |
-             +--------------------------------------+
-                 ^                       ^
-                 | low-level             | control
-                 | data exchange         | operations
-                 |                       |
-                 |                       v
-                 |                  +-----------+
-                 | AF_NFC           |  netlink  |
-                 | socket           +-----------+
-                 | raw                   ^
-                 |                       |
-                 v                       v
-             +---------+            +-----------+
-             | rawsock | <--------> |   core    |
-             +---------+            +-----------+
-                                         ^
-                                         |
-                                         v
-                                    +-----------+
-                                    |  driver   |
-                                    +-----------+
+.. code-block:: none
+
+        +--------------------------------------+
+        |              USER SPACE              |
+        +--------------------------------------+
+            ^                       ^
+            | low-level             | control
+            | data exchange         | operations
+            |                       |
+            |                       v
+            |                  +-----------+
+            | AF_NFC           |  netlink  |
+            | socket           +-----------+
+            | raw                   ^
+            |                       |
+            v                       v
+        +---------+            +-----------+
+        | rawsock | <--------> |   core    |
+        +---------+            +-----------+
+                                    ^
+                                    |
+                                    v
+                               +-----------+
+                               |  driver   |
+                               +-----------+
 
 Device Driver Interface
 =======================