edit installation az command

to install an az extension, you have to use the `add` command group.  The error here was that the initial installation command said to use update.  This is not correct.

Author: johnwildes

articles/aks/use-managed-identity.md

@@ -40,7 +40,7 @@ You must have the following resources installed:
 To install the aks-preview 0.4.14 extension or later, use the following Azure CLI commands:
 
 ```azurecli
-az extension update --name aks-preview
+az extension add --name aks-preview
 az extension list
 ```