Fix use of to_parquet in storage tutorial

[JulianWgs]

• 'has_nulls=True' was replaced with 'storage_options={"has_nulls": True}'
• engine was changed to "fastparquet"
  • avoids error: "TypeError: unhashable type: 'dict'"

Fix #124

[JulianWgs]

The TypeError relates to a new version of pyarrow, I think.

https://github.com/pandas-dev/pandas/issues/26121 https://stackoverflow.com/questions/52997610/python-pandas-describe-throwing-error-unhashable-type-dict

[TomAugspurger]

Thanks.

I think there's already a jira ticket for Arrow about the TypeError.