Update server.yaml

[1473371932]

Update some error comments

  ## flow_metrics database data retention time(unit: hour)
  #flow-log-ttl-hour:
  #  l4-flow-log: 72
  #  l7-flow-log: 72
  #  l4-packet: 72

After update

  ## flow_log database data retention time(unit: hour)
  #flow-log-ttl-hour:
  #  l4-flow-log: 72
  #  l7-flow-log: 72
  #  l4-packet: 72