fix broken contributing guide

[cmppoon]

What does this change

I fixed the Contributing Guide page by setting the type of the code blocks to bash, which resolved the formatting issue. I also moved the git commit command inside a code block to make the page more consistent.

What issue does it fix

Closes #2938

Notes for the reviewer

Please let me know if any changes are required.

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR

[schristoff]

Thank you so much for this contribution to our documentation :)