fix env error on README.md

[taiojia]

The correct env is AWS_ACCESS_KEY not AWS_S3_ACCESS_KEY, I have fixed the README.md

[dmp42]

This is an example config, in which one can use any env value they wish.

Now, being consistent with the default config file is certainly something desirable - but then, the "correct" env var is AWS_KEY (see https://github.com/docker/docker-registry/blob/master/config/config_sample.yml#L86)

[taiojia]

OK, Thanks! @dmp42