Update reference TOC

Author: Microsoft Graph DevX Tooling

api-reference/v1.0/toc/external-data-connections/toc.yml

@@ -90,6 +90,8 @@ items:
     href: ../../resources/externalconnectors-itemidresolver.md
   - name: Json
     href: ../../resources/json.md
+  - name: Principal
+    href: ../../resources/externalconnectors-principal.md
   - name: Properties
     href: ../../resources/externalconnectors-properties.md
   - name: Property