1) README.md update, 2) bug fix

[ddxor]

Updating README.md for new awscli output format (region is now parameter #3) and a minor bug fix in cli/build.go.

[dan-v]

@ddxor - thanks for the PR. Could you validate if the following works with your version of the AWS CLI (it does for me on older version aws-cli/2.0.39):
aws ec2 describe-regions --query "Regions[*].RegionName" --output text
If so, would be nice if you could pull that into your PR instead.

[ddxor]

@dan-v confirmed your new syntax works, and updated the PR to reflect.

Interestingly enough, I am running aws-cli/1.16.113, so it's quite possible that my change was necessary because my version was too old. However, this new method works across versions, so hurrah!