Update index.yml

The PDF builds for Azure are breaking due to an outdated schema for landing pages. 

This value is causing the failure:
      - linkListType: training

That is a valid value, but PDF doesn't know about it yet.
https://static.docs.com/ui/latest/schemas/Landing.schema.json

Trying the alternative to see if PDF will work:
      - linkListType: learn

Author: JasonWHowell

articles/virtual-network-manager/index.yml

@@ -33,7 +33,7 @@ landingContent:
             url: faq.md
           - text: Use cases
             url: concept-use-cases.md
-      - linkListType: training
+      - linkListType: learn
         links:
           - text: Create a hub-spoke network topology with Azure Virtual Network Manager
             url: /training/modules/hub-and-spoke-network-architecture/4a-create-hub-spoke-virtual-network-manager