Fix docs building for main
This PR reverts the triggering event for building documentation introduced by:
- #5250

Fix #5326.
_The documentation is not available anymore as the PR was closed or merged._ EDIT
At least the docs for ~~main~~ PR branch are now built:
- https://github.com/huggingface/datasets/actions/runs/3594847760/jobs/6053620813 Build documentation for main branch was triggered after this PR being merged: https://github.com/huggingface/datasets/actions/runs/3603370082/jobs/6071482470