[FIX] pos_restaurant : add missing t-ref

Current behavior:
When printing a bill in the restaurant you had an error poping.

Steps to reproduce:
- Open a restaurant session
- Go on a table and add some products
- Click on the bill button
- Click on the print button
- You get an error

opw-3259014

closes odoo/odoo#119041

X-original-commit: 1d1808e7136c6416fe88c28e7971e97707de2557
Signed-off-by: Heinz Robin (rhe) <rhe@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>

addons/pos_restaurant/static/src/xml/Screens/BillScreen.xml

@@ -16,7 +16,7 @@
                 </div>
                 <div class="centered-content">
                     <div class="button print" t-on-click="printReceipt">
-                        <i class="fa fa-print"></i>
+                        <i class="fa fa-print" t-ref="order-print-receipt-button"></i>
                         <span> </span>
                         <span>Print</span>
                     </div>