docs: Minor updates to poetry usage details & contributing.md

[emilyschultz]

Hello,

This is just a small PR for a couple of documentation updates:

• I ran across a doc typo in the poetry usage instructions, for the pyproject.toml file.
• And in the contribution guidelines, I updated the flag for signing off on commits.

Thanks!

[jkowalleck]

Thank you very much for fixing those errors, @emilyschultz .