chore(deps-dev): bump rexml in /docs in the bundler group

Bumps the bundler group in /docs with 1 update: [rexml](https://github.com/ruby/rexml).


Updates `rexml` from 3.3.8 to 3.3.9
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.8...v3.3.9)

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

Signed-off-by: dependabot[bot] <[email protected]>

docs/Gemfile.lock

@@ -242,7 +242,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rexml (3.3.8)
+    rexml (3.3.9)
     rouge (3.30.0)
     rubyzip (2.3.2)
     safe_yaml (1.0.5)