CloudFormation does not allow _ in the key only alphanumeric values

Good evening

Let me first say thank you for providing this tutorial

[nginx-reverse-proxy-sidecar-ecs-fargate-task](https://containersonaws.com/pattern/nginx-reverse-proxy-sidecar-ecs-fargate-task)

I have followed the tutorial and when it came to deploying using the command below:
```sh
sam deploy \
--template-file parent.yml \
--stack-name nginx-reverse-proxy \
--resolve-s3 \
--capabilities CAPABILITY_IAM \
--parameter-overrides AppImageUrl=$APP_URI NginxImageUrl=$NGINX_URI
```

I came across the following error
`Error: Invalid value for '--parameter-overrides': AppImageUrl= is not in valid format. It must look something like 'ParameterKey=KeyPairName,ParameterValue=MyKey ParameterKey=InstanceType,ParameterValue=t1.micro' or 'KeyPairName=MyKey InstanceType=t1.micro'
`

This article explains why:

[invalid-value-for-parameter-overrides](https://stackoverflow.com/questions/73438094/yaml-file-not-deploying-invalid-value-for-parameter-overrides)

Therefore I replaced the variables with`AppImageUrl=APP1` and `NginxImageUrl=NGINX1` and was able to deploy.

If this was what one is supposed to do, then the article does not state this or maybe things have change since the article was published. I thought I would mention in case anyone who is new to AWS might encounter this issue.

many thanks.

Numel