Fix train_unit_example.py (#547)

Summary:
Pull Request resolved: #547

This example was not updated after API changes

Reviewed By: ananthsub

Differential Revision: D49474480

fbshipit-source-id: 6c8f2165e2bfc66e0f9737e26218f17e25c40f7e

examples/train_unit_example.py

@@ -146,10 +146,9 @@ def main(argv: List[str]) -> None:
         args.log_every_n_steps,
     )
 
-    # pyre-fixme[28]: Unexpected keyword argument `dataloader`.
     train(
         my_unit,
-        dataloader=dataloader,
+        train_dataloader=dataloader,
         max_epochs=args.max_epochs,
     )