Correction of Custom Networking - Automate Configuration with Availability Zone Labels

**Describe the problem**

In step 2 of the [Automate Configuration with Availability Zone Labels](https://github.com/aws/aws-eks-best-practices/blob/0712b92843229f8cf7d7718a5b0d3e7e0a0cab02/content/networking/custom-networking/index.md?plain=1#L90) documentation, it recommends running this command to enable automatic configuration of the ENI config based on the AZ.

`kubectl set env daemonset aws-node -n kube-system AWS_VPC_K8S_CNI_CUSTOM_NETWORK_CFG=true`

I believe the command is meant to be:

`kubectl set env daemonset aws-node -n kube-system ENI_CONFIG_LABEL_DEF=topology.kubernetes.io/zone`

As it currently reads, the implication is that setting `AWS_VPC_K8S_CNI_CUSTOM_NETWORK_CFG=true` would be enough but I don't think that's true. Please correct me if I'm wrong.

**References**

Setting the ENI_CONFIG_LABEL_DEF is in line with the official AWS [docs](https://docs.aws.amazon.com/eks/latest/userguide/cni-custom-network.html#custom-networking-configure-kubernetes) on custom networking.

Open content/networking/custom-networking/index.md and review step 2 of “Automate Configuration with Availability Zone Labels.” Compare the documented command with the linked official AWS custom-networking documentation. Done means the step accurately explains and shows the required environment setting, with the surrounding instructions remaining consistent.

Written by the indexing model from the issue text.