Fixed a small problem with the alignment prerejective (tutorial) code

As pointed out by Sylvain in the mailing list (http://www.pcl-users.org/Alignment-prerejective-tutorial-td4028856.html), the alignment prerejective did not function consistently on all platforms. This is now solved by reducing the normal estimation radius, resulting in better FPFH matching, finally giving a correct alignment in the end.

Thanks to Sylvain!