doc: fix flow action command names in testpmd guide

doc: fix flow action command names in testpmd guide

testpmd actions set_tp_src and set_tp_dst documentation adds the
of_ prefix to action names, while the implementation doesn't add it.

This patch removes the prefix from action names in testpmd
documentation.

Fixes: 9ccc94919500 ("ethdev: add flow API actions to modify TCP/UDP port numbers")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>
Acked-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>