[AKS] fix documentation for linux_profile ssh keys (#3099)

Resolves #1940

From the [REST API documentation](https://docs.microsoft.com/en-us/rest/api/aks/managedclusters/createorupdate#containerservicesshconfiguration) only one SSH key is expected.  I have modified the documentation to reflect this.

azurerm/resource_arm_kubernetes_cluster.go

@@ -278,6 +278,7 @@ func resourceArmKubernetesCluster() *schema.Resource {
 							Type:     schema.TypeList,
 							Required: true,
 							ForceNew: true,
+							MaxItems: 1,
 
 							Elem: &schema.Resource{
 								Schema: map[string]*schema.Schema{

website/docs/d/kubernetes_cluster.html.markdown

@@ -147,7 +147,7 @@ A `linux_profile` block exports the following:
 
 * `admin_username` - The username associated with the administrator account of the managed Kubernetes Cluster.
 
-* `ssh_key` - One or more `ssh_key` blocks as defined below.
+* `ssh_key` - An `ssh_key` block as defined below.
 
 ---
 

website/docs/r/kubernetes_cluster.html.markdown

@@ -136,7 +136,7 @@ A `linux_profile` block supports the following:
 
 * `admin_username` - (Required) The Admin Username for the Cluster. Changing this forces a new resource to be created.
 
-* `ssh_key` - (Required) One or more `ssh_key` blocks. Changing this forces a new resource to be created.
+* `ssh_key` - (Required) An `ssh_key` block. Only one is currently allowed.  Changing this forces a new resource to be created.
 
 ---