README: Fix build flag description.
README: Fix build flag description.

StanfordLegion/legion,StanfordLegion/legion,sdalton1/legion,sdalton1/legion,sdalton1/legion,sdalton1/legion,chuckatkins/legion,chuckatkins/legion,StanfordLegion/legion,chuckatkins/legion,StanfordLegion/legion,sdalton1/legion,chuckatkins/legion,sdalton1/legion,StanfordLegion/legion,StanfordLegion/legion,chuckatkins/legion,chuckatkins/legion,StanfordLegion/legion,chuckatkins/legion,StanfordLegion/legion