[Python][Doc] Add import statement to `filters_to_expression` docstring example

### Describe the enhancement requested

The docstring example for `pyarrow.parquet.filters_to_expression` calls the function without any import statement.

https://github.com/apache/arrow/blob/8c27898d36f45ece9b0d114b3d6c6a42d20bde7f/python/pyarrow/parquet/core.py#L149

The example should reflect how users would actually call the function:

```python
>>> import pyarrow.parquet as pq
>>> pq.filters_to_expression([('foo', '==', 'bar')])

```

### Component(s)

Documentation