Fix AWS ECR authentication docs (#781)

The command you listed here did not work. The command from the official documentation did:

https://docs.aws.amazon.com/AmazonECR/latest/userguide/getting-started-cli.html

aws ecr get-login-password --region region | docker login --username AWS --password-stdin aws_account_id.dkr.ecr.region.amazonaws.com

README.md

@@ -219,7 +219,7 @@ aws ecr create-repository --repository-name k8s/kube-bench --image-tag-mutabilit
 ```
 git clone https://github.com/aquasecurity/kube-bench.git
 cd kube-bench
-aws ecr get-login-password --region <AWS_REGION> | docker login --username <AWS_USERNAME> --password-stdin <AWS_ACCT_NUMBER>.dkr.ecr.<AWS_REGION>.amazonaws.com
+aws ecr get-login-password --region <AWS_REGION> | docker login --username AWS --password-stdin <AWS_ACCT_NUMBER>.dkr.ecr.<AWS_REGION>.amazonaws.com
 docker build -t k8s/kube-bench .
 docker tag k8s/kube-bench:latest <AWS_ACCT_NUMBER>.dkr.ecr.<AWS_REGION>.amazonaws.com/k8s/kube-bench:latest
 docker push <AWS_ACCT_NUMBER>.dkr.ecr.<AWS_REGION>.amazonaws.com/k8s/kube-bench:latest
@@ -431,4 +431,4 @@ We welcome pull requests!
 - You're welcome to submit a draft PR if you would like early feedback on an idea or an approach.
 - Happy coding!
 
-[kube-bench-aws-security-hub]: ./docs/asff.md
+[kube-bench-aws-security-hub]: ./docs/asff.md