[Docs] Add language identifiers to fenced code blocks (#28955)

Add language identifiers to code blocks

docs/source/en/custom_models.md

@@ -310,7 +310,7 @@ Use `register_for_auto_class()` if you want the code files to be copied. If you
 you don't need to call it. In cases where there's more than one auto class, you can modify the `config.json` directly using the 
 following structure:
 
-```
+```json
 "auto_map": {     
 	"AutoConfig": "<your-repo-name>--<config-name>",     
 	"AutoModel": "<your-repo-name>--<config-name>",