Bump rexml from 3.3.6 to 3.3.9 in /docs (#1595)

Author: dependabot[bot]

docs/Gemfile.lock

@@ -55,13 +55,11 @@ GEM
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (3.22.0)
     safe_yaml (1.0.5)
     sassc (2.4.0)
       ffi (~> 1.9)
-    strscan (3.1.0)
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     unicode-display_width (1.7.0)