docs: networking: packet_mmap: fix formatting for C macros

The citation of macro definitions should appear in a code block.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/5cb47005e7a59b64299e038827e295822193384c.1609232919.git.baruch@tkos.co.ilSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/packet_mmap.rst

@@ -437,7 +437,7 @@ and the following flags apply:
 Capture process
 ^^^^^^^^^^^^^^^
 
-     from include/linux/if_packet.h
+From include/linux/if_packet.h::
 
      #define TP_STATUS_COPY          (1 << 1)
      #define TP_STATUS_LOSING        (1 << 2)