Needs AKS-preview CLI extension

Author: JackStromberg

articles/application-gateway/for-containers/quickstart-deploy-application-gateway-for-containers-alb-controller-addon.md

@@ -40,6 +40,7 @@ You need to complete the following tasks before enabling the ALB Controller add-
 
     # Install Azure CLI extensions.
     az extension add --name alb
+    az extension add --name aks-preview 
     ```
     
     # [Azure PowerShell](#tab/azure-powershell)