DOC/BUG: broken example in read_parquet with selecting columns
In the dev docs the example that subsets the columns to read with `read_parquet` is broken for the pyarrow engine: http://pandas-docs.github.io/pandas-docs-travis/io.html#io-parquet 

```
In [514]: result = pd.read_parquet('example_pa.parquet', engine='pyarrow', columns=['a', 'b'])
...
IndexError: Table column index 6 is out of range

In [515]: result = pd.read_parquet('example_fp.parquet', engine='fastparquet', columns=['a', 'b'])

In [516]: result.dtypes
Out[516]: 
a    object
b     int64
dtype: object
```

This is due to a bug in ``pyarrow`` (which I am reporting over there, due to how pyarrow deals with the pandas metadata if not all columns are present), but in the meantime we should also fix our docs to not show this buggy example.