doc: fix ring based driver guide

Correct the sample code in the pcap_ring.rst file to match the latest
rte_eth_ring.c code.

The parameters to the rte_eth_from_rings() function have changed since
the documentation was written.
The API change occurred before DPDK 1.8 when the rst files were added.
The original documentation on which the pcap_ring.rst file was based was
not correct.

Fixes: fc1f2750a3ec ("doc: programmers guide")

Signed-off-by: Bernard Iremonger <bernard.iremonger@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>