doc: cleanup multi-pthread prog guide
doc: cleanup multi-pthread prog guide

Fixes: 1733be6d3147 ("doc: new eal multi-pthread feature")

Reported-by: Siobhan Butler <8cf2da75f16c15b95e66f52a35c85d9ba5dc91d6@intel.com>
Signed-off-by: Cunming Liang <7edc6dd0948177c688da9b288939d80da3db4303@intel.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk