DOC: update the parquet docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
################################################################################
################### Docstring (pandas.DataFrame.to_parquet)  ###################
################################################################################

Write a DataFrame to the binary parquet format.

.. versionadded:: 0.21.0

Requires either fastparquet or pyarrow libraries.

Parameters
----------
fname : str
    String file path.
engine : {'auto', 'pyarrow', 'fastparquet'}, default 'auto'
    Parquet library to use. If 'auto', then the option
    ``io.parquet.engine`` is used. The default ``io.parquet.engine``
    behavior is to try 'pyarrow', falling back to 'fastparquet' if
    'pyarrow' is unavailable.
compression : {'snappy', 'gzip', 'brotli', None}, default 'snappy'
    Name of the compression to use. Use ``None`` for no compression.
kwargs : dict
    Additional keyword arguments passed to the engine.

Examples
----------
>>> df = pd.DataFrame(data={'col1': [1, 2], 'col2': [3, 4]})
>>> df.to_parquet('df.parquet.gzip', compression='gzip')

Returns
----------
None

See Also
--------
DataFrame.to_csv : write a csv file.
DataFrame.to_sql : write to a sql table.
DataFrame.to_hdf : write to hdf.

################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.DataFrame.to_parquet" correct. :)
```