markdown_preview: Fix release notes title being overridden (#31703)

Closes: #31701

Screenshot:

<img width="383" alt="image"
src="https://github.com/user-attachments/assets/7fd8ce70-2208-4aca-bc70-860d6c649765"
/>



Release Notes:

- Fixed in-app release notes having an incorrect title

---------

Co-authored-by: Gilles Peiffer <gilles.peiffer.yt@gmail.com>

crates/auto_update_ui/src/auto_update_ui.rs

@@ -91,7 +91,7 @@ fn view_release_notes_locally(
 
                             let buffer = cx.new(|cx| MultiBuffer::singleton(buffer, cx));
 
-                            let tab_content = SharedString::from(body.title.to_string());
+                            let tab_content = Some(SharedString::from(body.title.to_string()));
                             let editor = cx.new(|cx| {
                                 Editor::for_multibuffer(buffer, Some(project), window, cx)
                             });