docs: improve minor issues within DPDK L3fwd user guide

- Improve syntax highlighting
- Remove unused MLNX_OFED Software Download link.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: Ib5a849b8e7fa94ccafc7ab8f6e4e6426ea4e5b24