[release/v14] Fix typo in rancher2_cluster public_access_sources docs

Backport #1592 to release/v14 for #2117

Please add this issue to the proper milestone.
Copied from PR: 
The docs say the type of this variable is a `map` but it actually is a list of strings. See: https://github.com/rancher/terraform-provider-rancher2/blob/master/rancher2/schema_cluster_eks_config_v2.go#L249
