[cloudsql-instance] Add cloudsql_iam_authentication flag to fix example in readme (#3069)

Co-authored-by: Luca Prete <lucaprete@google.com>

modules/cloudsql-instance/README.md

@@ -129,6 +129,7 @@ module "db" {
   tier             = "db-g1-small"
 
   flags = {
+    cloudsql_iam_authentication    = "on"
     disconnect_on_expired_password = "on"
   }