As far as I'm aware there isn't a way of changing the context menu not to display the file name to be scanned. The only thing I see is not to display the icon, but the 'scan' file name is still present, so that is no help.
I recall this being asked once before and the official answer was no there wasn't a way to do this. You could add it to the Wishlist Topic. The problem is if fixed as just a scan with avast! there are likely to be those who like the file name being displayed, so you can't please everyone. I don't know if they could have an extra program setting under the Explorer Extension, not to display the file name.