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>

Documentation/networking/devlink/devlink-trap.rst

@@ -257,6 +257,8 @@ drivers:
   * :doc:`netdevsim`
   * :doc:`mlxsw`
 
+.. _Generic-Packet-Trap-Groups:
+
 Generic Packet Trap Groups
 ==========================