Bad circup instructions

[Neradoc]

The circup instructions in the readme are generated as such:

circup install {{ cookiecutter.library_name | lower | replace(" ", "-") }}

This leads to instructions like: circup install ducky or circup install ltr329-ltr303

• it should use underscores and not dash in the library name
• it is missing the library prefix: adafruit_ (or other if present)