PR from @mausy5043 

This pull request proposes two changes:
1. Fix small typos in `CONTRIBUTING.md`
2. In `update.sh` give precedence to `.bz2` and `.gz` files over `.xz`. Reason for this is that I noticed that on some platforms (e.g. OSX) the `xz-utils` is not natively installed. `bzip2` and `gzip` are more commonly supported.