Bump rexml from 3.3.6 to 3.3.9 in /docs (#866)

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.6 to 3.3.9.

docs/Gemfile.lock

@@ -67,16 +67,14 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (4.2.0)
     safe_yaml (1.0.5)
     sass-embedded (1.77.4)
       google-protobuf (>= 3.25, < 5.0)
       rake (>= 13)
     sass-embedded (1.77.4-x86_64-darwin)
       google-protobuf (>= 3.25, < 5.0)
-    strscan (3.1.0)
     terminal-table (3.0.2)
       unicode-display_width (>= 1.1.1, < 3)
     unicode-display_width (2.5.0)