DOC: Update index parameter in pandas to_parquet
- [ ] closes #xxxx (reference: https://github.com/python-sprints/pandas-mentoring/issues/156)
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

The documentation ([https://dev.pandas.io/reference/api/pandas.DataFrame.to_parquet.html#pandas.DataFrame.to_parquet](https://dev.pandas.io/reference/api/pandas.DataFrame.to_parquet.html#pandas.DataFrame.to_parquet)) says that for `DataFrame.to_parquet` when the value for the index parameter is `None`, the behavior will depend on the engine. However if we try this both engines, `pyarrow` and `fastparquet`, will keep the index. This PR proposes to set the default value of the index parameter as `index=True` instead of `index=None` since both engines keep the index anyway. Would appreciate any discussion/feedback :)

Sample code to demonstrate how both engines treat the index:
```
import pandas as pd 
  
data = {'name':['a', 'b', 'c', 'd']} 
df = pd.DataFrame(data, index =['rank_1', 'rank_2', 'rank_3', 'rank_4'])

df.info()
```
Output:
```
<class 'pandas.core.frame.DataFrame'>
Index: 4 entries, rank_1 to rank_4
Data columns (total 1 columns):
name    4 non-null object
dtypes: object(1)
memory usage: 64.0+ bytes
```
```
# pyarrow
df.to_parquet("test_pyarrow.parquet", engine="pyarrow")
df_read_pyarrow = pd.read_parquet("test_pyarrow.parquet", engine="pyarrow")
df_read_pyarrow.info()
```
Output:
```
<class 'pandas.core.frame.DataFrame'>
Index: 4 entries, rank_1 to rank_4
Data columns (total 1 columns):
name    4 non-null object
dtypes: object(1)
memory usage: 64.0+ bytes
```

```
# fastparquet
df.to_parquet("test_fastparquet.parquet", engine="fastparquet")
df_read_fastparquet = pd.read_parquet("test_fastparquet.parquet", engine="fastparquet")
df_read_fastparquet.info()
```
Output:
```
<class 'pandas.core.frame.DataFrame'>
Index: 4 entries, rank_1 to rank_4
Data columns (total 1 columns):
name    4 non-null object
dtypes: object(1)
memory usage: 64.0+ bytes
```