Fix syntax warning with terraform 0.13 (#35)

Terraform 0.13 will throw the following warning:

> Warning: Interpolation-only expressions are deprecated
>   on .terraform/modules/dbpool.deploy_nixos/deploy_nixos/main.tf line
>   104, in data "external" "nixos-instantiate":
>    104:     "--argstr", "system", "${var.target_system}"
>    Terraform 0.11 and earlier required all non-constant expressions to be
>    provided via interpolation syntax, but this pattern is now deprecated. To
>    silence this warning, remove the "${ sequence from the start and the }"
> sequence from the end of this expression, leaving just the inner expression.
> Template interpolation syntax is still used to construct strings from
> expressions when the template includes multiple interpolation sequences or a
> mixture of literal strings and interpolations. This deprecation applies only
> to templates that consist entirely of a single interpolation sequence.
> (and 3 more similar warnings elsewhere)

Since this syntax is already supported in 0.12 and everybody should have
upgraded to that till now, this change should be safe.

Author: fpletz

aws_image_nixos/README.md

@@ -18,7 +18,7 @@ supported.
     }
 
     resource "aws_instance" "example" {
-      ami           = "${module.nixos_image_1903.ami}"
+      ami           = module.nixos_image_1903.ami
       instance_type = "t2.micro"
 
       ...

google_image_nixos/README.md

@@ -22,7 +22,7 @@ resource "google_compute_instance" "example" {
 
   boot_disk {
     initialize_params {
-      image = "${module.nixos_image_1809.self_link}"
+      image = module.nixos_image_1809.self_link
     }
   }