[Qt] fix missing plural form for a string in sendcoinsdialog

- this was reported on Transifex by pryds (Thank you!)

src/qt/sendcoinsdialog.cpp

@@ -631,7 +631,7 @@ void SendCoinsDialog::updateSmartFeeLabel()
     {
         ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), feeRate.GetFeePerK()) + "/kB");
         ui->labelSmartFee2->hide();
-        ui->labelFeeEstimation->setText(tr("Estimated to begin confirmation within %1 block(s).").arg(nBlocksToConfirm));
+        ui->labelFeeEstimation->setText(tr("Estimated to begin confirmation within %n block(s).", "", nBlocksToConfirm));
     }
 
     updateFeeMinimizedLabel();