Add a no-GAM chunk to the Cactus example

[adamnovak]

This adds a no-GAM chunk to test simplifying pre-made chunks (although it doesn't really have small variants to simplify out).

I also fixed the docs and the script, because they hadn't been exercised and were wrong.

You can make another chunk like this with:

cd exampleData
../scripts/prepare_chunks.sh -x cactus.vg.xg -r ref:500-600 -d 'region without read tracks' -o chunk-cactus-no-reads >> cactus.bed