I was on the [Visual Customiztions - Editor
Scrollbar](https://zed.dev/docs/visual-customization#editor-scrollbar)
section of the docs, and copy and pasted the code block into my personal
Zed settings and saw there was a syntax error.

This is a PR to add a missing comma and fix the syntax error in the
docs.

First time contributing, please let me know if I missed any
steps/important info.

Release Notes:

- N/A