docs-networking: fix typo in define

The #define for NETIF_F_GSO_UDP_L4 was incorrect in the
documentation, fix it by making it match the actual code.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>