doc: update FAQ with new testpmd default burst size
doc: update FAQ with new testpmd default burst size

Fixes: 836853d3d4cf ("app/testpmd: increase default burst size to 32")

Signed-off-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

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