fix(cookiecutter): adapter_type to lowercase

[qleroy]

Summary

Fixes #441.

This makes the adapter_type in cookiecutter.json to lowercase so that it matches the adapter directory names and make the script post_gen_project.sh work.

Testing instructions

$ git clone https://github.com/betodealmeida/shillelagh.git
$ cd shillelagh
$ python -m venv venv
$ source venv/bin/activate
$ python -m pip install cookiecutter
$ cookiecutter templates/adapter/

[betodealmeida]

Thanks for fixing this!