doc: fix -n option in FreeBSD guide
doc: fix -n option in FreeBSD guide

Fix EAL usage to indicate that -n is, now, optional.

Signed-off-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

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