docs: Improve display of inline-diagnostics config sample

Closes helix-editor#11591

book/src/editor.md

@@ -452,7 +452,8 @@ fn main() {
 
 The new diagnostic rendering is not yet enabled by default. As soon as end of line or inline diagnostics are enabled the old diagnostics rendering is automatically disabled. The recommended default setting are:
 
-```
+```toml
+[editor]
 end-of-line-diagnostics = "hint"
 [editor.inline-diagnostics]
 cursor-line = "warning" # show warnings and errors on the cursorline inline