Update README.md

[thjonson]

version in example needed to be updated to at least 2.1.2 to resolve iksVersionName and policyName error:

$ terraform plan
╷
│ Warning: Experimental feature "module_variable_optional_attrs" is active
│
│ on .terraform/modules/terraform-intersight-iks/versions.tf line 10, in terraform:
│ 10: experiments = [module_variable_optional_attrs]
│
│ Experimental features are subject to breaking changes in future minor or patch releases, based on feedback.
│
│ If you have feedback on the design of this feature, please open a GitHub issue to discuss it.
│
│ (and 3 more similar warnings elsewhere)
╵
╷
│ Error: Missing required argument
│
│ on .terraform/modules/terraform-intersight-iks/main.tf line 116, in module "k8s_version":
│ 116: module "k8s_version" {
│
│ The argument "iksVersionName" is required, but no definition was found.
╵
╷
│ Error: Missing required argument
│
│ on .terraform/modules/terraform-intersight-iks/main.tf line 116, in module "k8s_version":
│ 116: module "k8s_version" {
│
│ The argument "policyName" is required, but no definition was found.
╵
╷
│ Error: Unsupported argument
│
│ on .terraform/modules/terraform-intersight-iks/main.tf line 119, in module "k8s_version":
│ 119: k8s_version = var.version_policy.version
│
│ An argument named "k8s_version" is not expected here.
╵
╷
│ Error: Unsupported argument
│
│ on .terraform/modules/terraform-intersight-iks/main.tf line 120, in module "k8s_version":
│ 120: k8s_version_name = var.version_policy.name
│
│ An argument named "k8s_version_name" is not expected here.