app/flow-perf: fix typo in usage help
app/flow-perf: fix typo in usage help

From hairping-rss into hairpin-rss.

Fixes: bf3688f1e816 ("app/flow-perf: add insertion rate calculation")

Signed-off-by: Wisam Jaddo <cbdf453c8bf861f460ccde953a2ad4b2bcf4ed16@mellanox.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk