Bump league/commonmark from 2.3.8 to 2.3.9 (#977)

Bumps [league/commonmark](https://github.com/thephpleague/commonmark) from 2.3.8 to 2.3.9.
- [Release notes](https://github.com/thephpleague/commonmark/releases)
- [Changelog](https://github.com/thephpleague/commonmark/blob/2.3/CHANGELOG.md)
- [Commits](thephpleague/commonmark@2.3.8...2.3.9)

---
updated-dependencies:
- dependency-name: league/commonmark
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>