Remove docstring saying dfk.submit returns DataFutures. It does not. (#…

…3662)

## Type of change

- Update to human readable text: Documentation/error messages/comments
- Code maintenance/cleanup

parsl/dataflow/dflow.py

@@ -987,7 +987,7 @@ def submit(self,
             - app_kwargs (dict) : Rest of the kwargs to the fn passed as dict.
 
         Returns:
-               (AppFuture) [DataFutures,]
+            AppFuture
 
         """