use private URLs in EC2 registry post processors HCL example

[joshwizzy]

Context

The post processors examples in the Amazon EC2 Container Registry section are inconsisent:

• The JSON example uses a private registry URL: 12345.dkr.ecr.us-east-1.amazonaws.com/packer
• The HCL2 uses a public URL: public.ecr.aws/YOUR REGISTRY ALIAS HERE/YOUR REGISTRY NAME HERE

Changes

Update the URLs in the HCL2 example to match the JSON example

[hashicorp-cla]

All committers have signed the CLA.

[lbajolet-hashicorp]

FYI @joshwizzy,

I re-generated the docs based on your changes (they're generated with make generate from the file you modified) since the generation checks complained, and pushed that change on top of your branch.

I'll merge the PR once it's green