Documentation/networking: Update Intel Wired LAN docs

- Update the Intel Wired LAN documentation with the latest
  URL for ethtool.

- replace "Ethtool" with "ethtool"

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>