missing def keyword

Signed-off-by: Frost Ming <[email protected]>

bindings/python/README.md

@@ -24,7 +24,7 @@ Or using the async API:
 ```python
 import asyncio
 
-async main():
+async def main():
     op = opendal.AsyncOperator("fs", root="/tmp")
     await op.write("test.txt", "Hello World")
     print(await op.read("test.txt"))