Documentation: Use latest Terraform syntax

[Stretch96]

This updates the documentation to Terraform version 0.11+ syntax, which allows constant expressions to be provided without the interpolation syntax

Current documentation snippet:

output "google_addrs" {
  value = "${join(",", data.dns_a_record_set.google.addrs)}"
}

Terraform output:

Warning: Interpolation-only expressions are deprecated

  on test.tf line 71, in output "google_addrs":
  71:   value = "${join(",", data.dns_a_record_set.google.addrs)}"

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.

[kellystuard]

Not a reviewer in the repo. But, looks good to me.

[hashicorp-cla]

All committers have signed the CLA.

[austinvalle]

Apologies on the delay, but these docs have since been updated/adjusted since we migrated to Plugin Framework in #260, thanks!

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