build(deps): bump rexml from 3.3.9 to 3.4.2 in /docs/v3

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.9 to 3.4.2.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.9...v3.4.2)

---
updated-dependencies:
- dependency-name: rexml
  dependency-version: 3.4.2
  dependency-type: indirect
...

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

Author: dependabot[bot]

docs/v3/Gemfile.lock

@@ -126,7 +126,7 @@ GEM
     rb-inotify (0.11.1)
       ffi (~> 1.0)
     redcarpet (3.6.1)
-    rexml (3.3.9)
+    rexml (3.4.2)
     rouge (3.30.0)
     sass (3.7.4)
       sass-listen (~> 4.0.0)