fix example (#178)

quickstart/201-function-app-key-vault-ref/variables.tf

@@ -1,10 +1,11 @@
-variable "name_prefix" {
+variable "prefix" {
   type        = string
+  default     = "func-app-kvref"
   description = "Prefix of the resource name"
 }
 
 variable "location" {
   type        = string
+  default     = "eastus"
   description = "Location to deploy the resource group"
-  default     = "West US 2"
-}
+}