AboutImageJ: use $IJ/images/about, not $IJ/about
AboutImageJ: use $IJ/images/about, not $IJ/about

This will allow us to start shipping the about images using the ImageJ
Updater, without needing to hardcode a special case "about" folder.

This is part of the fix for Fiji issue #115:
  https://github.com/fiji/fiji/issues/115

imagej/imagej-common