Fix redirect of Classic UI demo site

Author: stevepiercy

docs/install/index.md

@@ -26,7 +26,7 @@ In this part of the documentation, you can find how to try Plone and how to choo
 Choose a version.
 
 -   [Plone 6 with Volto frontend](https://6.demo.plone.org/)
--   [Plone 6 Classic UI (nightly build)](https://6-classic.demo.plone.org/)
+-   [Plone 6 Classic UI (nightly build)](https://6-classic.demo.plone.org/en)
 -   [Plone 5.2.x (stable) with Barceloneta frontend](https://demo.plone.org/)
 :::