Correct Ruby target version in README.md
Correct Ruby target version in README.md

Follows up changes in c577df3bb3e1f3320036ed976a0fdf5cf410e21e

empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core,empirical-org/Empirical-Core