Update UserAssignedIdentity providerID with azure:// prefix in templates

Author: nojnhuh

docs/book/src/topics/workload-identity.md

@@ -142,7 +142,7 @@ to give the identity Contributor access to the Azure subscription where the work
   ```yaml
   identity: UserAssigned
   userAssignedIdentities:
-  - providerID: /subscriptions/${AZURE_SUBSCRIPTION_ID}/resourceGroups/${RESOURCE_GROUP}/providers/Microsoft.ManagedIdentity/userAssignedIdentities/${USER_ASSIGNED_IDENTITY_NAME}
+  - providerID: azure:///subscriptions/${AZURE_SUBSCRIPTION_ID}/resourceGroups/${RESOURCE_GROUP}/providers/Microsoft.ManagedIdentity/userAssignedIdentities/${USER_ASSIGNED_IDENTITY_NAME}
   ```
 
   A sample `AzureMachineTemplate` after the edit should look like the below:
@@ -162,7 +162,7 @@ to give the identity Contributor access to the Azure subscription where the work
         sshPublicKey: ${AZURE_SSH_PUBLIC_KEY_B64:=""}
         identity: UserAssigned
         userAssignedIdentities:
-        - providerID: /subscriptions/${AZURE_SUBSCRIPTION_ID}/resourceGroups/${RESOURCE_GROUP}/providers/Microsoft.ManagedIdentity/userAssignedIdentities/${USER_ASSIGNED_IDENTITY_NAME}
+        - providerID: azure:///subscriptions/${AZURE_SUBSCRIPTION_ID}/resourceGroups/${RESOURCE_GROUP}/providers/Microsoft.ManagedIdentity/userAssignedIdentities/${USER_ASSIGNED_IDENTITY_NAME}
         vmSize: ${AZURE_NODE_MACHINE_TYPE}
   ```