Remove preview tag

Author: Tanu Balla

articles/expressroute/TOC.yml

@@ -136,7 +136,7 @@
       href: expressroute-howto-set-global-reach.md
     - name: Azure CLI
       href: expressroute-howto-set-global-reach-cli.md
-  - name: Add IPv6 support for private peering (Preview)
+  - name: Add IPv6 support for private peering
     items:
     - name: Azure portal
       href: expressroute-howto-add-ipv6-portal.md