Fix README.md by replacing go.py with 'env.py go'.
Fix README.md by replacing go.py with 'env.py go'.

go.py no longer exists.

TBR=iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/596513004

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra