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

docs/src/diagnostics.md

@@ -51,7 +51,7 @@ To configure, use
 
 ```json5
 "project_panel": {
-  "diagnostics": "all",
+  "show_diagnostics": "all",
 }
 ```