Bump rexml from 3.3.9 to 3.4.2 in /docs (#639)

Author: dependabot[bot]

docs/Gemfile.lock

@@ -59,7 +59,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.3.9)
+    rexml (3.4.2)
     rouge (3.30.0)
     safe_yaml (1.0.5)
     sassc (2.4.0)