Bump rexml from 3.2.5 to 3.2.8 in /docs

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 to 3.2.8.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.5...v3.2.8)

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

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

docs/Gemfile.lock

@@ -67,14 +67,16 @@ GEM
  rb-fsevent (0.10.4)
  rb-inotify (0.10.1)
  ffi (~> 1.0)
- rexml (3.2.5)
+ rexml (3.2.8)
+ strscan (>= 3.0.9)
  rouge (3.21.0)
  safe_yaml (1.0.5)
  sass (3.7.4)
  sass-listen (~> 4.0.0)
  sass-listen (4.0.0)
  rb-fsevent (~> 0.9, >= 0.9.4)
  rb-inotify (~> 0.9, >= 0.9.7)
+ strscan (3.1.0)
  thread_safe (0.3.6)
  tzinfo (1.2.10)
  thread_safe (~> 0.1)