Add missing region entries to .aws/credentials file`

I had an error trying to run an ansible playbook because of a configuration error with AWS, and we solved it by adding the region field in all entries of the credentials file

tf/README.md

@@ -11,12 +11,15 @@ Terraform is used for managing the OONI infrastructure as code.
 [oonidevops_user]
 aws_access_key_id = YYYY
 aws_secret_access_key = ZZZ
+region = eu-central-1
 [oonidevops_user_dev]
 role_arn = arn:aws:iam::905418398257:role/oonidevops
 source_profile = oonidevops_user
+region = eu-central-1
 [oonidevops_user_prod]
 role_arn = arn:aws:iam::471112720364:role/oonidevops
 source_profile = oonidevops_user
+region = eu-central-1
 ```
 
 Where you replace OONI_ORG_ID with the ID of the ORG you are deploying to (dev,