Fix markdown preview heading overflows no wrap (#8052)

![Kapture 2024-02-20 at 18 27
15](https://github.com/zed-industries/zed/assets/8416130/87d4dcea-e2f0-44ba-88a4-06829dbb0e89)

Release Notes:

- Improved markdown preview wrapping ([#8047](https://github.com/zed-industries/zed/issues/8047)).