For request path for option in documentation

This commit fixes the request path in the `federation_whitelist_endpoint_enabled` config option.

The path was changed towards the tail end of development in #16848.

docs/usage/configuration/config_documentation.md

@@ -1236,7 +1236,7 @@ federation_domain_whitelist:
 
 Enables an endpoint for fetching the federation whitelist config.
 
-The request method and path is `GET /_synapse/client/config/federation_whitelist`, and the
+The request method and path is `GET /_synapse/client/v1/config/federation_whitelist`, and the
 response format is:
 
 ```json