Merge pull request #402 from treefrogframework/dependabot/bundler/docs/rexml-3.3.9

Bump rexml from 3.3.6 to 3.3.9 in /docs

Author: treefrogframework

docs/Gemfile.lock

@@ -240,8 +240,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (3.30.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
@@ -256,7 +255,6 @@ GEM
       faraday (>= 0.17.3, < 3)
     simpleidn (0.2.1)
       unf (~> 0.1.4)
-    strscan (3.1.0)
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     textutils (1.4.0)