Updated 2019 toc with link to 2020.

Author: Heidilohr

articles/virtual-desktop/virtual-desktop-fall-2019/TOC.yml

@@ -4,6 +4,8 @@
   items:
   - name: What is Windows Virtual Desktop?
     href: ../overview.md
+  - name: Spring 2020 Release documentation
+    href: ./create-host-pools-azure-marketplace.md
 - name: Tutorials
   items:
   - name: 1. Create a tenant