doc: fix QinQ flow rules in testpmd guide
doc: fix QinQ flow rules in testpmd guide

In the Testpmd Flow rules management section, correct
the TPID values in the Sample QinQ flow rules sub section.
Also replace the keyword qinq_strip with extend in the
vlan set command.

Fixes: bef3bfe7d5f4 ("doc: revise sample testpmd flow commands")
Cc: stable@dpdk.org

Signed-off-by: Bernard Iremonger <233eeff376ad0c06f66f06c5522ea3c4feb30d3b@intel.com>
Acked-by: Xiaoyun Li <151cc00bdba86cfa2ecfc39aaa98def8f092d9d8@intel.com>

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