ox: Fix Italian translation for "Figure"

* lisp/ox.el (org-export-dictionary): Fix Italian translation for
  "Figure".

Reported-by: Franco Pasut <franco.pasut@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2018-06/msg00373.html>

lisp/ox.el

@@ -5824,6 +5824,7 @@ them."
      ("es" :default "Figura")
      ("et" :default "Joonis")
      ("is" :default "Mynd")
+     ("it" :default "Figura")
      ("ja" :default "図" :html "図")
      ("no" :default "Illustrasjon")
      ("nb" :default "Illustrasjon")