doc: fix a typo in EAL guide
doc: fix a typo in EAL guide

The correct name for virt2memseg API is `rte_mem_virt2memseg`, not
`rte_virt2memseg`.

Fixes: 950e8fb4e194 ("mem: allow registering external memory areas")
Cc: stable@dpdk.org

Signed-off-by: Anatoly Burakov <814e00d8f6e4e81b7ccc7e3e5a0bfec0078f0203@intel.com>

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