examples/flow_filtering: fix example documentation

examples/flow_filtering: fix example documentation

Previous patch removed the VLAN item from example code.
This patch fixes the code and documentation accordingly.

Code update includes fix of comments, and removal of redundant
variables and their initialization.
Documentation update reflects the code changes done in previous
patch and in this patch.

Fixes: 9af4eb565710 ("examples/flow_filtering: remove VLAN item")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>