MND-5: Fix operating system requirements

Do not mention point releases in the Operating System Requirements
section of the Reference Architecture document.

Change-Id: Ic63f7beb0599868cd962b906b36e140f2acfc02e
Fixes: MND-5
Signed-off-by: Jan Hilberath <jan@midokura.com>
Merge "MND-5: Fix operating system requirements"