docs: Update settings in diagnostics.md (#36806)

For project_panel, the diagnostics key seems to be `show_diagnostics`
not `diagnostics`
([source](https://github.com/zed-industries/zed/blob/main/crates/project_panel/src/project_panel_settings.rs#L149-L152)).
Updating the docs accordingly

Release Notes:

- N/A
d49409cabaitsaphel committed on 8/23/2025, 4:11:27 PM· committed by GitHubparent60ea475
1 file changedLine totals unavailable