update header level

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: skarim

docs/src/content/docs/guides/ui.md

@@ -77,7 +77,7 @@ Before a PR in the stack can be merged, the following conditions must be met:
 
 ![Merge box for a stacked pull request](../../../assets/screenshots/stack-merge-box.png)
 
-## Rebasing from the UI
+### Rebasing from the UI
 
 When the stack is not linear (e.g., after changes were pushed to a lower branch, or after `main` has moved ahead), a **Rebase Stack** button appears in the merge box. Clicking it triggers a server-side cascading rebase that: