CONTRIBUTING.md references setup.py instead of poetry

**Describe the bug**
I was trying to get started so I followed the instructions in `CONTRIBUTING.md`.

At line 71, it refers to `setup.py` which was deleted last August. I think the correct instruction is `poetry build`.

**To Reproduce**
```
Paste the command(s) you ran and the output.
If there was a crash, please include the traceback here.
```

** Environment (please complete the following information):**
- OS: Windows 11 running an Ubuntu 20 WSL environment.
- Package Version: 0.4.0

**Additional context**
Add any other context about the problem here.