Fix function schema reference in activity_as_tool

Author: AnatoliB

samples-v2/openai_agents/tools.py

@@ -35,7 +35,7 @@ async def run_activity(ctx: RunContextWrapper[Any], input: str) -> Any:
         return result
 
     schema = function_schema(
-        func=run_activity,
+        func=activity_func._function._func,
         name_override=activity_name,
         docstring_style=None,
         description_override=None,