Don't describe LKE Cluster label as optional

[0xch4z]

If no label is provided, one will be assigned automatically.

This bit is inaccurate. If no label is provided, the API throws a Bad Request error.

❯ curl --location --request POST 'https://api.linode.com/v4/lke/clusters' \
	--header 'Content-Type: application/json' \
	--header "Authorization: Bearer $LINODE_API_TOKEN" \
	--data-raw '{
	    "k8s_version": "1.17",
	    "region": "us-east",
	    "node_pools": [
	        {
	            "type": "g6-standard-1",
	            "count": 1
	        }
	    ]
	}'
{"errors": [{"reason": "label is required", "field": "label"}]}