[Python] test failure in io.rst

`make pytest` gives me the following error:
```
**********************************************************************
File "io.rst", line 511, in default
Failed example:
import pyarrow as pa
import pyarrow.dataset as ds

ds.write_dataset(data, "./partitioned", format="parquet",
partitioning=ds.partitioning(pa.schema([("year", pa.int16())])))
Exception raised:
Traceback (most recent call last):
File "/home/antoine/miniconda3/envs/cookbook/lib/python3.9/doctest.py", line 1336, in __run
exec(compile(example.source, filename, "single",
File "", line 4, in
ds.write_dataset(data, "./partitioned", format="parquet",
File "/home/antoine/miniconda3/envs/cookbook/lib/python3.9/site-packages/pyarrow/dataset.py", line 878, in write_dataset
_filesystemdataset_write(
File "pyarrow/_dataset.pyx", line 3408, in pyarrow._dataset._filesystemdataset_write
File "pyarrow/error.pxi", line 99, in pyarrow.lib.check_status
pyarrow.lib.ArrowInvalid: Could not write to ./partitioned as the directory is not empty and existing_data_behavior is to error
**********************************************************************
```

Start by running `make pytest` and inspect the example at line 511 of `io.rst`, where the dataset-writing doctest fails because `./partitioned` is not empty. Check how this example is prepared and rerun the doctest after the relevant documentation or test setup is adjusted. Done means the `io.rst` doctest passes under `make pytest`.

Written by the indexing model from the issue text.