Bump rexml from 3.2.8 to 3.3.3 in /docs (#3965)

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.8 to 3.3.3.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.8...v3.3.3)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: indirect
...

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