In doc https://docs.openshift.com/container-platform/4.16/machine_management/control_plane_machine_management/cpmso_provider_configurations/cpmso-config-options-aws.html#cpmso-yaml-failure-domain-aws_cpmso-config-options-aws

1. Should change 
<cluster_id>-private-<aws_zone_a> 
to 
<cluster_id>-subnet-private-<aws_zone_a>

2. 5 "Specifies the subnet name for an additional failure domain, using the infrastructure ID and the AWS availability zone" change to "Specifies an AWS availability zone for an additional failure domain."

3. 6 "Specifies the cluster's infrastructure ID and the AWS availability zone for the additional failure domain" change to "Specifies the subnet name for an additional failure domain, using the infrastructure ID and the AWS availability zone."