Fix docs validation to detect unknown keys (#49660)

Closes #ISSUE

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>

docs/src/languages/cpp.md

@@ -180,7 +180,7 @@ Allows switching between corresponding C++ source files (e.g., `.cpp`) and heade
 by running the command {#action editor::SwitchSourceHeader} from the command palette or by setting
 a keybinding for the `editor::SwitchSourceHeader` action.
 
-```json [settings]
+```json [keymap]
 {
   "context": "Editor",
   "bindings": {