Fix btnInstallNexmonFirmware text

[francozappa]

Hi,

First of all, thanks for the great project and app!

I think that you can improve a bit the app layout of the Firmware installation view. This is just a small fix, however you can easily improve the backup and restore text and logic. In my understanding the backup is created even if there is already a backup file and a new backup file is create with a.bac appended. Additionally, it is not clear from the screen which backup is restored when there are multiple backups. What I did to restore the old firmware was to manually select it and click the Install Nexmon Firmware.