Bump rexml from 3.3.9 to 3.4.2 in /docs (#657)

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Author: dependabot[bot]

docs/Gemfile.lock

@@ -222,7 +222,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.3.9)
+    rexml (3.4.2)
     rouge (3.26.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)