fixed annotation

google/cloud/bigtable/data/_async/client.py

@@ -279,7 +279,7 @@ def _build_grpc_channel(self, *args, **kwargs) -> SwappableChannelType:
         Returns:
           a custom wrapped swappable channel
         """
-        create_channel_fn: Callable[[], Any]
+        create_channel_fn: Callable[[], Channel]
         if self._emulator_host is not None:
             # emulators use insecure channel
             create_channel_fn = partial(insecure_channel, self._emulator_host)