minor doc fix
minor doc fix

`spark_model` is an instance of `elephas.spark_model.SparkModel`.
According to the source code, `SparkModel.train` doesn't have argument `num_workers`.
Ref: https://github.com/maxpumperla/elephas/blob/9a394d7f7486dbf5ceefd121447c01b3c44c941e/elephas/spark_model.py#L188
maxpumperla/elephas,maxpumperla/elephas