org-exp.el: Add russian translation.

TINYCHANGE

Thanks to Vladimir Lomov for this change.

lisp/org-exp.el

@@ -199,6 +199,7 @@ This option can also be set with the +OPTIONS line, e.g. \"-:nil\"."
     ("nb" "Forfatter"  "Dato"  "Innhold" "Fotnoter")  ;; nb = Norsk (bokm.l)
     ("nn" "Forfattar"  "Dato"  "Innhald" "Fotnotar")  ;; nn = Norsk (nynorsk)
     ("pl" "Autor"      "Data" "Spis treści"  "Przypis")
+    ("ru" "Автор"      "Дата"  "Содержание" "Сноски")
     ("sv" "Författare" "Datum" "Innehåll" "Fotnoter"))
   "Terms used in export text, translated to different languages.
 Use the variable `org-export-default-language' to set the language,