Use MyST syntax for link.

Author: stevepiercy

docs/backend/upgrading/v60.md

@@ -55,7 +55,7 @@ The classic integrated frontend which is called "Classic UI" is still available
 If you choose to use Classic UI, then do not use the `plone.volto` package.
 ```
 
-To learn how to upgrade a existing site to run on Volto please read [Upgrading to Volto](backend-upgrade-to-volto-label)
+To learn how to upgrade an existing Plone site to run with Volto, please read {doc}`upgrade-to-volto`.
 
 
 See [PLIP 2703](https://github.com/plone/Products.CMFPlone/issues/2703).