Fix learn_bpe.lua option example
Fix learn_bpe.lua option example

Fixes #201

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