fix retrieving unused coupons, apply byteweaver#35

Author: Vlsarro

coupons/admin.py

@@ -98,7 +98,7 @@ def num_coupons_used(self, obj):
     num_coupons_used.short_description = _("used")
 
     def num_coupons_unused(self, obj):
-        return obj.coupons.used().count()
+        return obj.coupons.unused().count()
 
     num_coupons_unused.short_description = _("unused")