Fix example argument parser in docs (#1692)
Fix example argument parser in docs (#1692)

[`parser.parse_known_args()`](https://docs.python.org/3.7/library/argparse.html#argparse.ArgumentParser.parse_known_args) actually returns a tuple of the Namespace of known args and a list of unknown args. We only want the former.
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning