No need to interpolate anymore

Terraform 0.13 came up with some preferred shorter syntax for this, so
let's make sure we don't get any warnings anymore.

Author: franckverrot

main.tf

@@ -10,7 +10,7 @@ terraform {
 variable "stripe_api_token" {} # populate this by exporting TF_VAR_stripe_api_token
 
 provider "stripe" {
-  api_token = "${var.stripe_api_token}"
+  api_token = var.stripe_api_token
 }
 
 resource "stripe_product" "my_product" {
@@ -30,21 +30,21 @@ resource "stripe_product" "my_product_with_id" {
 }
 
 resource "stripe_plan" "my_product_plan" {
-  product  = "${stripe_product.my_product.id}"
+  product  = stripe_product.my_product.id
   amount   = 12345
   interval = "month" # day week month year
   currency = "usd"
 }
 
 resource "stripe_plan" "free_product_plan" {
-  product  = "${stripe_product.free_product.id}"
+  product  = stripe_product.free_product.id
   amount   = 0
   interval = "month"
   currency = "usd"
 }
 
 resource "stripe_plan" "my_product_metered_plan" {
-  product  = "${stripe_product.my_product.id}"
+  product  = stripe_product.my_product.id
   interval = "month"
   currency = "usd"
 
@@ -72,21 +72,21 @@ resource "stripe_plan" "my_product_metered_plan" {
 resource "stripe_plan" "my_product_plan_with_id" {
   plan_id = "my_plan"
 
-  product  = "${stripe_product.my_product.id}"
+  product  = stripe_product.my_product.id
   amount   = 3232
   interval = "month" # day week month year
   currency = "usd"
 }
 
 resource "stripe_plan" "my_decimal_product_plan" {
-  product        = "${stripe_product.my_product.id}"
+  product        = stripe_product.my_product.id
   amount_decimal = 123.45
   interval       = "month" # day week month year
   currency       = "usd"
 }
 
 resource "stripe_plan" "my_transformed_product_plan" {
-  product  = "${stripe_product.my_product.id}"
+  product  = stripe_product.my_product.id
   amount   = 2401
   interval = "month" # day week month year
   currency = "usd"
@@ -108,7 +108,7 @@ resource "stripe_webhook_endpoint" "my_endpoint" {
 
 output "webhook_secret" {
   sensitive = true
-  value     = "${stripe_webhook_endpoint.my_endpoint.secret}"
+  value     = stripe_webhook_endpoint.my_endpoint.secret
 }
 
 resource "stripe_coupon" "mlk_day_coupon_25pc_off" {
@@ -142,7 +142,7 @@ resource "stripe_price" "my_price" {
     blm = "always"
   }
   nickname    = "my price"
-  product     = "${stripe_product.my_product.id}"
+  product     = stripe_product.my_product.id
   unit_amount = 1337
   recurring = {
     interval       = "month"