adding preview to toc

Author: daphnemamsft

articles/governance/resource-graph/toc.yml

@@ -79,7 +79,7 @@
   - name: Guidance for throttled requests
     displayName: pagination, batching, throttling, parallel
     href: ./concepts/guidance-for-throttled-requests.md
-  - name: Guidance for the Azure Resource Graph GET/LIST API
+  - name: Guidance for the Azure Resource Graph GET/LIST API (Preview)
     href: ./concepts/azure-resource-graph-get-list-api.md
   - name: Security
     items: