build(deps): bump rexml from 3.2.5 to 3.2.8 in /docs

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 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@v3.2.5...v3.2.8)

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

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

docs/Gemfile.lock

@@ -75,7 +75,8 @@ GEM
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.5)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
     rouge (3.22.0)
     safe_yaml (1.0.5)
     sass (3.7.4)
@@ -88,6 +89,7 @@ GEM
     sawyer (0.8.2)
       addressable (>= 2.3.5)
       faraday (> 0.8, < 2.0)
+    strscan (3.1.0)
     thread_safe (0.3.6)
     tzinfo (1.2.10)
       thread_safe (~> 0.1)