Fix minor typo in option description
Fix minor typo in option description

OpenNMT/OpenNMT,da03/OpenNMT,jsenellart-systran/OpenNMT,jsenellart-systran/OpenNMT,jsenellart/OpenNMT,jungikim/OpenNMT,monsieurzhang/OpenNMT,da03/OpenNMT,monsieurzhang/OpenNMT,jsenellart-systran/OpenNMT,OpenNMT/OpenNMT,da03/OpenNMT,monsieurzhang/OpenNMT,jungikim/OpenNMT,jsenellart/OpenNMT,jungikim/OpenNMT,jsenellart/OpenNMT,OpenNMT/OpenNMT