samples: net: zperf: Update the README file

Clarify what iPerf version was used when testing this sample
application. It was iPerf 2.0.9 which worked ok, for example
iPerf 2.0.13 did not work properly with UDP uploader (client)
mode.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>