Update documentation for target_regions block

[dhananjaipai]

Overview of the Issue

The documentation for target_regions does not specify the expected inputs in the block. It just shows

target_region ([]TargetRegion) - A target region to store the image version in. ...

with no reference to what []TargetRegion is.

Reproduction Steps

Try setting target_region based on official docs and it does not work

The PR #387 seems to use replicas instead of regionalReplicaCount or regional_replica_count as expected, and there is no reference to this in the documentation.

Plugin and Packer version

From packer version 1.11.1