fixed keygen script to reflect changes in aws-cli

[thesophiaxu]

the current aws-cli version (installed from latest pip3 repository) outputs regions like this:
haojixu@haojis-MacBook-Pro ~ % aws ec2 describe-regions --output text REGIONS ec2.eu-north-1.amazonaws.com opt-in-not-required eu-north-1 REGIONS ec2.ap-south-1.amazonaws.com opt-in-not-required ap-south-1 REGIONS ec2.eu-west-3.amazonaws.com opt-in-not-required eu-west-3 REGIONS ec2.eu-west-2.amazonaws.com opt-in-not-required eu-west-2 REGIONS ec2.eu-west-1.amazonaws.com opt-in-not-required eu-west-1 REGIONS ec2.ap-northeast-2.amazonaws.com opt-in-not-required ap-northeast-2 REGIONS ec2.ap-northeast-1.amazonaws.com opt-in-not-required ap-northeast-1 REGIONS ec2.sa-east-1.amazonaws.com opt-in-not-required sa-east-1 REGIONS ec2.ca-central-1.amazonaws.com opt-in-not-required ca-central-1 REGIONS ec2.ap-southeast-1.amazonaws.com opt-in-not-required ap-southeast-1 REGIONS ec2.ap-southeast-2.amazonaws.com opt-in-not-required ap-southeast-2 REGIONS ec2.eu-central-1.amazonaws.com opt-in-not-required eu-central-1 REGIONS ec2.us-east-1.amazonaws.com opt-in-not-required us-east-1 REGIONS ec2.us-east-2.amazonaws.com opt-in-not-required us-east-2 REGIONS ec2.us-west-1.amazonaws.com opt-in-not-required us-west-1 REGIONS ec2.us-west-2.amazonaws.com opt-in-not-required us-west-2

so the script should be print $4 instead of print $3.

[dan-v]

Thanks for the PR! Bonus points if you can make it backwards compatible :)