calendar: Fix number formatting in some languages

Some languages require using a conversion modifier to display the day
number in a translated shape. fa_IR is affected for example.

https://bugzilla.gnome.org/show_bug.cgi?id=673235

js/ui/calendar.js

@@ -725,7 +725,7 @@ const Calendar = new Lang.Class({
         // nRows here means 6 weeks + one header + one navbar
         let nRows = 8;
         while (row < 8) {
-            let button = new St.Button({ label: iter.getDate().toString(),
+            let button = new St.Button({ label: iter.toLocaleFormat(C_("date day number format", "%d")),
                                          can_focus: true });
             let rtl = button.get_text_direction() == Clutter.TextDirection.RTL;