fix example (#167)

quickstart/101-windows-vm-with-iis-server/variables.tf

@@ -3,11 +3,8 @@ variable "resource_group_location" {
   description = "Location of the resource group."
 }
 
-variable "resource_group_name_prefix" {
-  default     = "rg"
-  description = "Prefix of the resource group name that's combined with a random ID so name is unique in your Azure subscription."
-}
-
-variable "admin_password" {
-  description = "The password to assign to the Windows Administrator user account."
+variable "prefix" {
+  type        = string
+  default     = "win-vm-iis"
+  description = "Prefix of the resource name"
 }