docs: networking: devlink-trap: Fix reference to other document

This fixes the following Sphinx warning:

Documentation/networking/devlink-trap.rst:175: WARNING: unknown document: /devlink-trap-netdevsim

Fixes: 9e087457 ("Documentation: Add description of netdevsim traps")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>