[Shipper Pro] Add a description to the final Shipper screen

At the last screen of Shipper, I was just about to click Begin Migration and I suddenly couldn’t remember if I’d clicked “Import” or “Export” when I started. So, I cancelled it and started over.

It would be clearer if there was something that said, “Click to Import <site-1> into <site-2> and overwrite <site-2>” or “Click to Export <site-1> to <site-2> and overwrite <site-2>”.