Fix typos (#57)

src/mainwindow.cpp

@@ -646,7 +646,7 @@ void MainWindow::backupWalletDat() {
     QFile wallet(QDir(rpc->getConnection()->config->zcashDir).filePath("wallet.dat"));
     if (!wallet.exists()) {
         QMessageBox::critical(this, "No wallet.dat", "Couldn't find the wallet.dat on this computer."
-            "You need to backup it up from the machine zcashd is running on", QMessageBox::Ok);
+            "You need to back it up from the machine zcashd is running on", QMessageBox::Ok);
         return;
     }
     
@@ -658,7 +658,7 @@ void MainWindow::backupWalletDat() {
 
     if (!wallet.copy(backupName.toLocalFile())) {
         QMessageBox::critical(this, "Couldn't backup", "Couldn't backup the wallet.dat file."
-            "You need to backup it up manually.", QMessageBox::Ok);
+            "You need to back it up manually.", QMessageBox::Ok);
     }
 }