Bump rexml from 3.2.5 to 3.3.9 in /docs (#95)

Author: lildude

docs/Gemfile.lock

@@ -245,7 +245,7 @@ GEM
     rb-fsevent (0.11.1)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.5)
+    rexml (3.3.9)
     rouge (3.26.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)