Updating vanilla_pad_features.rst with proper class name

The old documentation said:

   from bob.pad.base.database import CSVPADDataset

   database = CSVPADDataset("path/to/my_dataset", "my_protocol")

However, CSVPADDataset does not exist in bob.pad.base.dabase and it should be FileListPadDatabase instead. See https://gitlab.idiap.ch/bob/bob.pad.base/-/blob/master/bob/pad/base/database/csv_dataset.py#L35`FileListPadDatabase`