Invalid example AWS policy for configuring S3 media_store

[jakecoppinger]

The example S3 policy at https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/docs/configuring-playbook-s3.md appears invalid. This is the error returned from the Edit bucket policy form in the AWS Console:

Unknown Error
An unexpected error occurred.
API response
Missing required field Principal

I'm not sure what the value of Principal should be. I really don't want to get this wrong as I believe this allows public access to the bucket if configured incorrectly.

The AWS docs are at https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_elements_principal.html

Should "Principal": { "AWS": "..." } be added where ... is the user's account ID? (in the dropdown under the username in the top right of the AWS Console).