Bump rexml from 3.2.9 to 3.3.9 in /demo

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.9 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@v3.2.9...v3.3.9)

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

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

Author: dependabot[bot]

demo/Gemfile.lock

@@ -2,10 +2,10 @@ GEM
   remote: https://rubygems.org/
   specs:
     ast (2.4.2)
-    auto_html (2.1.1)
+    auto_html (2.2.0)
       gemoji (~> 4.0.0.rc2)
       redcarpet (~> 3.5)
-      rexml (~> 3.2.5)
+      rexml (~> 3.3.9)
       rinku (~> 2.0)
     did_you_mean (2.0.0)
     diff-lcs (1.5.1)
@@ -47,8 +47,7 @@ GEM
     regexp_parser (2.10.0)
     rerun (0.14.0)
       listen (~> 3.0)
-    rexml (3.2.9)
-      strscan
+    rexml (3.3.9)
     rinku (2.0.6)
     roda (3.87.0)
       rack
@@ -78,7 +77,6 @@ GEM
     rubocop-ast (1.37.0)
       parser (>= 3.3.1.0)
     ruby-progressbar (1.13.0)
-    strscan (3.1.2)
     tilt (2.5.0)
     unicode-display_width (3.1.3)
       unicode-emoji (~> 4.0, >= 4.0.4)