examples/flow_filtering: enhance code snippet readability

examples/flow_filtering: enhance code snippet readability

The 'IPv4' in the comment is to mark the code snippet, while it made
some confusion. Then removing 'IPv4' description will be clearer.

Fixes: 9a212dc06c7a ("doc: use code snippets in sample app guides")

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Conor Walsh <conor.walsh@intel.com>