update toc

Author: mumian

articles/azure-resource-manager/bicep/toc.yml

@@ -740,5 +740,7 @@ items:
       href: diagnostics/bcp401.md
     - name: BCP414
       href: diagnostics/bcp414.md
+    - name: BCP420
+      href: diagnostics/bcp420.md
   - name: Azure CLI
     href: /cli/azure/resource