correct statement about CPU utilization and GRE

- according to discussion on mailing list
  (https://lists.launchpad.net/fuel-dev/msg01084.html) usage of GRE
  protocol *increases* CPU utilization on endpoint nodes, our
  documentation states the opposite, which is wrong.

Change-Id: Ia2f835a68c7ecbf812fb916a143d50423f126d40