fix: Fixed launch_templates_with_managed_node_group example (#1599)

examples/launch_templates_with_managed_node_groups/variables.tf

@@ -1,6 +1,6 @@
-variable "instance_type" {
-  description = "Instance type"
+variable "instance_types" {
+  description = "Instance types"
   # Smallest recommended, where ~1.1Gb of 2Gb memory is available for the Kubernetes pods after ‘warming up’ Docker, Kubelet, and OS
-  type    = string
-  default = "t3.small"
+  type    = list(string)
+  default = ["t3.small"]
 }