Update README.md (#2173)
Summary:
Updating the command to run python filr from python filename.py to python3 filename.py
Pull Request resolved: https://github.com/pytorch/benchmark/pull/2173
Reviewed By: aaronenyeshi
Differential Revision: D54092833
Pulled By: xuzhao9
fbshipit-source-id: f6ea9efd6e00f46e72d5134496ff74db62c23e87
