docs: Remove duplicate `indent_guides` setting in Project panel options (#26927)

Just a small docs pull request to remove the duplicate `indent_guides`
setting in the Project panel options section of the docs. There is also
another `indent_guides` on
[L2585](https://github.com/zed-industries/zed/pull/26927/files#diff-c2decf822f3085926bd23bdf175700222dfd8193d48ea39908d4bb1d1a7c6240R2584).
~~Let me know if you prefer to remove the other instance, thank you!~~

Edit: I just realized it's the old setting, `indent_guides` now expects
a object.