fix 101-cosmos-db-azure-container-instance example (#159)

quickstart/101-cosmos-db-azure-container-instance/variables.tf

@@ -3,7 +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 "prefix" {
+  type        = string
+  default     = "cosmos-db-aci"
+  description = "Prefix of the resource name"
 }