fix docs

Author: DavidBadura

docs/mkdocs.yml

@@ -113,7 +113,7 @@ nav:
     - CLI: cli.md
   - Supported Versions: supported-versions.md
   - Our BC Promise: our-backward-compatibility-promise.md
-  - Upgrade from 2.x: UPGRADE-3.0.md
+  - Upgrade from 3.x: UPGRADE-4.0.md
   - Links:
     - Blog: https://patchlevel.de/blog
     - Symfony Bundle: https://patchlevel.github.io/event-sourcing-bundle-docs/latest/