Documentation: devlink: fix broken link warning

At 'make htmldocs' the following warning is thrown:

Documentation/networking/devlink/devlink-trap.rst:302:
WARNING: undefined label: generic-packet-trap-groups

Fixes the warning by setting the label to the specified header,
within the same document.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>