Switch to a _raw_ download to allow automated download

[BenjaminHCCarr]

Changing:
https://github.com/dcppc/crosscut-metadata/blob/master/make-crosscut-instance-bdbag.sh
to
https://raw.githubusercontent.com/dcppc/crosscut-metadata/master/make-crosscut-instance-bdbag.sh

Allows a curl/wget of this needed script.

[jonathancrabtree]

Hi Ben! This particular script isn't much use without the rest of the code in the repo and if someone does a git clone (or downloads any of the releases) to obtain that code they will also get a copy of the script. I'd also point out that we don't expect too many people (if any) to attempt to run the script themselves: it is there primarily to document how the instance was generated. In other words, making this a raw URL makes it no easier for a sophisticated user to obtain a local copy of the script and if it makes it easier for an unsophisticated user to obtain such a copy without the code on which it depends, then that's probably a bad thing...