Minor doc issue in to_parquet(engine='auto')
#### Code Sample, a copy-pastable example if possible

```python
In [7]: pd.DataFrame.to_parquet?
Signature: pd.DataFrame.to_parquet(self, fname, engine='auto', compression='snappy', **kwargs)
Docstring:
Write a DataFrame to the binary parquet format.

.. versionadded:: 0.21.0

Parameters
----------
fname : str
    string file path
engine : {'auto', 'pyarrow', 'fastparquet'}, default 'auto' <--- here
```

but with `auto`:

```python

In [9]: pd.DataFrame([1,2,3]).to_parquet('f', engine='auto')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-9-057ba592ccd7> in <module>()
----> 1 pd.DataFrame([1,2,3]).to_parquet('f', engine='auto')

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/frame.py in to_parquet(self, fname, engine, compression, **kwargs)
   1647         from pandas.io.parquet import to_parquet
   1648         to_parquet(self, fname, engine,
-> 1649                    compression=compression, **kwargs)
   1650
   1651     @Substitution(header='Write out the column names. If a list of strings '

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/io/parquet.py in to_parquet(df, path, engine, compression, **kwargs)
    140     """
    141
--> 142     impl = get_engine(engine)
    143
    144     if not isinstance(df, DataFrame):

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/io/parquet.py in get_engine(engine)
     27
     28     if engine not in ['pyarrow', 'fastparquet']:
---> 29         raise ValueError("engine must be one of 'pyarrow', 'fastparquet'")
     30
     31     if engine == 'pyarrow':

ValueError: engine must be one of 'pyarrow', 'fastparquet'
```

#### Problem description

Either allow `auto` or don't have it in the docs & default

#### Expected Output

#### Output of ``pd.show_versions()``

<details>

In [10]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 17.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.5.0
Cython: 0.27.1
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: 0.10.0rc1-2-gf83361c
IPython: 6.2.1
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: 2.6.4
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: 1.3.0
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: 1.1.14
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: 0.2.0+8.g9eb9d77
pandas_datareader: None
</details>
