Add missing 'await writer.drain()' call to example (GH-29162)

Automerge-Triggered-By: GH:asvetlov

Doc/library/asyncio-stream.rst

@@ -352,6 +352,7 @@ TCP echo client using the :func:`asyncio.open_connection` function::
 
         print(f'Send: {message!r}')
         writer.write(message.encode())
+        await writer.drain()
 
         data = await reader.read(100)
         print(f'Received: {data.decode()!r}')