Bump rexml in /docs in the bundler group across 1 directory

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


Updates `rexml` from 3.3.6 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@v3.3.6...v3.3.9)

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

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

docs/Gemfile.lock

@@ -65,8 +65,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (4.3.0)
     safe_yaml (1.0.5)
     sass-embedded (1.69.5)
@@ -76,7 +75,6 @@ GEM
       google-protobuf (~> 3.23)
     sass-embedded (1.69.5-x86_64-darwin)
       google-protobuf (~> 3.23)
-    strscan (3.1.0)
     terminal-table (3.0.2)
       unicode-display_width (>= 1.1.1, < 3)
     unicode-display_width (2.5.0)