Custom report name in openerp

Question

When I print my report, it display the name of the report like this

for selection of one record

       "Delivery Slip-OUT_00057.pdf" 

for selection of two record

       "Delivery Slip-OUT_00057-OUT_00056.pdf"

SO for me, it is enough to show only

       "Delivery Slip.pdf"  

In both the cases, without editing in the base class

 <?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
    <report
        id="stock.report_picking_list_out" 
        model="stock.picking.out" 
        name="stock.picking.list.out" 
        string="Delivery Slip" 
        rml="XXXXX_stock_ext/report/delivery_slip.rml"
    />
</data>