Use realistic document-id value in docs example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: magicmark

docs/usage/extensions.rst

@@ -26,7 +26,7 @@ You can use this to pass protocol extensions such as
         request = GraphQLRequest(
             "query { viewer { name } }",
             extensions={
-                "document-id": "foo",
+                "document-id": "155d6e8f5545...",
             },
         )