docs/kubernetes_cluster: Fix inaccurate kube_config_raw description

[radeksimko]

The config we expose actually isn't base64 encoded as demonstrated below:

$ echo 'azurerm_kubernetes_cluster.test.kube_config_raw' | terraform console
apiVersion: v1
clusters:
- cluster:
    certificate-authority-data:
...

Whether it should be encoded is a separate discussion I'm happy to engage in, but the aim here is to at least bring docs up to date 😉

[tombuildsstuff]

LGTM 👍

[ghost]

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