docs: improve minor issues within VPP L3fwd user guide

- Provide missing command before sample output
- Improve syntax highlighting
- Provide missing ip neighbor command for suggested experiment

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: I6d56449e4e6dfd7f3c1807fbcf80d48fdb7b8a5e