chore(ha_vpn): Use external "URL" for source in the README (#34)
chore(ha_vpn): Use external "URL" for source in the README (#34)

Terraform recommends using an "external" URL for the source attribute in
a module in the README.

> Because examples will often be copied into other repositories for
> customization, any module blocks should have their source set to
> the address an external caller would use, not to a relative path.
-- https://www.terraform.io/docs/modules/index.html

terraform-google-modules/terraform-google-vpn,terraform-google-modules/terraform-google-vpn