Bump rexml from 3.2.5 to 3.3.9 in /docs

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 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@v3.2.5...v3.3.9)

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

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

Author: dependabot[bot]

docs/Gemfile.lock

@@ -50,7 +50,7 @@ GEM
     rb-fsevent (0.11.0)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.5)
+    rexml (3.3.9)
     rouge (3.26.0)
     safe_yaml (1.0.5)
     sassc (2.4.0)
@@ -60,6 +60,7 @@ GEM
     unicode-display_width (1.7.0)
 
 PLATFORMS
+  x86_64-linux
   x86_64-linux-musl
 
 DEPENDENCIES