Update `Running in an EKS cluster` documentation (#621)

Co-authored-by: Neha Viswanathan <nviswanathan@axway.com>
Co-authored-by: Liz Rice <liz@lizrice.com>

README.md

@@ -216,7 +216,7 @@ 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 --no-include-email --region <AWS_REGION>)
+aws ecr get-login-password --region <AWS_REGION> | docker login --username <AWS_USERNAME> --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