Rename dataset_infos.json to dataset_info.json

This was actually the right name. `dataset_infos.json` is used to have the infos of all the dataset configurations.

On the other hand `dataset_info.json` (without 's') is a cache file with the info of one specific configuration.

To fix #228, we probably just have to clear and reload the nlp-viewer cache.
As the file required for the viewing in the live nlp viewer is named as  dataset_info.json