Update swahili_news dataset
Please note also: the HuggingFace version at https://huggingface.co/datasets/swahili_news is outdated. An updated version, with deduplicated text and official splits, can be found at https://zenodo.org/record/5514203.

## Adding a Dataset
- **Name:** swahili_news

Instructions to add a new dataset can be found [here](https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md).

Related to:
- bigscience-workshop/data_tooling#107
