No need to use a personal access token anymore for Github Pages

As discussed in the [recent comments](https://github.community/t/github-action-not-triggering-gh-pages-upon-push/16096/28) in this page: https://github.community/t5/GitHub-Actions/Github-action-not-triggering-gh-pages-upon-push/td-p/26869, it is not needed anymore to use a personal access token to push to Github Pages, and the standard GitHub Token works.
I also [tested it](https://github.com/giovannipizzi/disk-objectstore/pull/42) and indeed this seems to work.

I suggest, therefore, to simplify the README removing the need of setting up a personal access token.

BTW, thanks for the great work on this action!