dpdk: Fix l3fwd command and typo changes in quick start guide

Fix the DPDK l3fwd command and add the requirement of root privileges.
Add the setup step, run and test section headings. Fix few typos.

Signed-off-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Nathan Brown <Nathan.Brown@arm.com>

Change-Id: Iebae259cee36b0b8f2f4b45a7270143ab4f5e744