Bump rexml from 3.3.3 to 3.3.6 in /docs in the bundler group

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


Updates `rexml` from 3.3.3 to 3.3.6
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.3...v3.3.6)

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

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

docs/Gemfile.lock

@@ -239,7 +239,7 @@ GEM
  rb-fsevent (0.11.2)
  rb-inotify (0.10.1)
  ffi (~> 1.0)
- rexml (3.3.3)
+ rexml (3.3.6)
  strscan
  rouge (3.30.0)
  ruby2_keywords (0.0.5)