fixes a minor bug and replaces an example

hexagongrid.pl:
if the side-length option were changed then the first row would have an
incorrect offset from the remaining rows. this change fixes this bug.

example.pl is based on koikexample.pl, but whereas koikexample.pl
sucked, example.pl makes a nice looking picture and the example is
fairly clear. This replacement improves the scant/non-existant docs.