chore(deps): bump rexml from 3.2.8 to 3.3.2 in /slate-docs (#459)

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.8 to 3.3.2.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.8...v3.3.2)

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

slate-docs/Gemfile.lock

@@ -98,8 +98,8 @@ GEM
     rb-inotify (0.10.1)
       ffi (~> 1.0)
     redcarpet (3.6.0)
-    rexml (3.2.8)
-      strscan (>= 3.0.9)
+    rexml (3.3.2)
+      strscan
     rouge (3.30.0)
     sass (3.7.4)
       sass-listen (~> 4.0.0)