Better to add timeout arguments for aws download

[WarBean]

With the suggested command in README to download the tar files, I frequently encountered a "connection closed" error after several minutes of downloading. According to this issue, I add an extra argument, then everything goes well:

aws s3 --no-sign-request --cli-connect-timeout 6000 cp s3://open-images-dataset/tar/test.tar.gz .

It is better to add --cli-connect-timeout 6000 into README so other people will not encounter the "connection closed" error again.