docs(emr): fix example datetime outputs for AWS CLI v2

[mjfxjas]

Fixes #9827.

The EMR command reference examples currently show Unix epoch timestamps for *DateTime fields; AWS CLI v2 renders these as ISO 8601 strings.

This updates the EMR example .rst files to use ISO 8601 output across the affected commands:

• create-security-configuration
• describe-cluster
• describe-step
• list-clusters
• list-instance-fleets
• list-instances
• list-security-configurations

[mjfxjas]

Friendly follow-up on this docs fix for EMR example datetime outputs in AWS CLI v2.

I kept the change narrow and surgical to match current behavior and improve copy-paste reliability for users. Happy to adjust quickly if maintainers prefer different phrasing or placement.