PR #547: from firedrakeproject/dependabot/bundler/docs/rexml-3.3.6

Bump rexml from 3.3.3 to 3.3.6 in /docs

Author: tommbendall

docs/Gemfile.lock

@@ -222,7 +222,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.3.3)
+    rexml (3.3.6)
       strscan
     rouge (3.26.0)
     ruby2_keywords (0.0.5)