Bump rexml from 3.3.3 to 3.3.9 in /docs

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.3 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@v3.3.3...v3.3.9)

---
updated-dependencies:
- dependency-name: rexml
  dependency-version: 3.3.9
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/Gemfile.lock

@@ -49,8 +49,7 @@ GEM
     rb-fsevent (0.10.3)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
-    rexml (3.3.3)
-      strscan
+    rexml (3.3.9)
     rouge (3.3.0)
     ruby_dep (1.5.0)
     safe_yaml (1.0.4)
@@ -59,7 +58,6 @@ GEM
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
-    strscan (3.1.0)
 
 PLATFORMS
   ruby