toc fix

Author: bwren

articles/azure-monitor/toc.yml

@@ -279,7 +279,7 @@ items:
           - name: Custom fields
             href: logs/custom-fields.md
           - name: Migrate from custom fields
-            href: logs/custom-fields.md
+            href: logs/custom-fields-migrate.md
       - name: Azure Diagnostics extension
         items:
         - name: Overview