fix reference generation for qr code

app/controllers/bills_controller.rb

@@ -14,9 +14,9 @@ class BillsController < ApplicationController
         throw :invalid_char
       end
     end
-    str2 = "#{str}2715"
+    str2 = "#{str}271500"
     rem = str2.to_i % 97
-    diff = (rem-1) % 97
+    diff = 98 - rem
     "RF#{'%02d' % diff}#{key}"
   end