[IMP] payment_mollie_official: set model description

Without this description logs & tests will gives a warning saying:
"2021-04-11 11:33:45,654 261297 WARNING your_database odoo.models: The model mollie.voucher.line has no _description"
By setting one this no longer pollutes the logs

payment_mollie_official/models/voucher_lines.py

@@ -9,6 +9,7 @@
 
 class MollieVoucherLines(models.Model):
     _name = 'mollie.voucher.line'
+    _description = 'Mollie Voucher Line'
 
     def _default_voucher_category(self):
         """ We moved field from acquirer to method line.