Update ArrowWriter path in examples
`ArrowWriter` is no longer in the top-level namespace in `datasets` v2.0 (see https://github.com/huggingface/datasets/pull/3875#discussion_r822924405), so I'm replacing the`datasets.ArrowWriter` occurances with `datasets.arrow_writer.ArrowWriter`.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Both are research projects that have `dataets` pinned to version 1.0.0 in their requirements, so I think they should be left as is.