Add missing machine type to Kubernetes provider

[ludelafo]

The documentation states a list of common machine types: https://registry.terraform.io/providers/iterative/iterative/latest/docs/resources/task#machine-type and https://github.com/iterative/terraform-provider-iterative/blob/main/docs/guides/generic-machine-types.md

I did notice the Kubernetes provider is missing the s machine type from the list. This pull request fixes this.