Update map type to Terraform 0.12 syntax

Author: karlkfi

examples/instance_with_attached_disk/variables.tf

@@ -52,7 +52,7 @@ variable "zone" {
 }
 
 variable "additional_metadata" {
-  type        = "map"
+  type        = map(string)
   description = "Additional metadata to attach to the instance"
   default     = {}
 }