docs/resource/aws_instance: Updated example Ubuntu AMI 14.04 -> 20.04 and variable interpolation to 0.12 syntax  (#13748)
docs/resource/aws_instance: Updated example Ubuntu AMI 14.04 -> 20.04 and variable interpolation to 0.12 syntax  (#13748)

* Updated example AMI from Ubuntu 14.04 to 20.04

* Updated comment to reflect code

* Updated variable interpolation syntax to 0.12
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws