Fix minor buffer search bar design issues (#7715)

This PR fixes the buffer search bar design issues mentioned in #7703.

It doesn't affect the project search bar.

Changes:

<img width="943" alt="zed-search-bar-design-issues"
src="https://github.com/zed-industries/zed/assets/2101250/af3bd0da-36cb-46ee-9af6-6b69911863d0">

Release Notes:

- N/A