docs: Add rest label the_canonical_patch_format

In preparation to convert Documentation/network/netdev-FAQ.rst to
restructured text format.  We would like to be able to reference 'the
canonical patch format' section.

Add rest label: 'the_canonical_patch_format'.
Signed-off-by: NTobin C. Harding <me@tobin.cc>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/process/submitting-patches.rst

@@ -611,6 +611,7 @@ which stable kernel versions should receive your fix. This is the preferred
 method for indicating a bug fixed by the patch. See :ref:`describe_changes`
 for more details.
 
+.. _the_canonical_patch_format:
 
 14) The canonical patch format
 ------------------------------