docs: Add missing ':' (#21751)

Release Notes:

- N/A

docs/src/languages/cpp.md

@@ -77,7 +77,7 @@ You can trigger formatting via {#kb editor::Format} or the `editor: format` acti
 
 ```json
   "languages": {
-    "C++" {
+    "C++": {
       "format_on_save": "on",
       "tab_size": 2
     }