Bump rexml from 3.2.6 to 3.2.8

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.6 to 3.2.8.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.6...v3.2.8)

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

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

Gemfile.lock

@@ -167,7 +167,8 @@ GEM
  rdf (~> 3.3)
  rexml (~> 3.2)
  redcarpet (3.6.0)
- rexml (3.2.6)
+ rexml (3.2.8)
+ strscan (>= 3.0.9)
  scanf (1.0.0)
  shacl (0.4.1)
  json-ld (~> 3.3)
@@ -196,6 +197,7 @@ GEM
  net-http-persistent (~> 4.0, >= 4.0.2)
  rdf (~> 3.3)
  stringio (3.1.0)
+ strscan (3.1.0)
  sxp (1.3.0)
  matrix (~> 0.4)
  rdf (~> 3.3)