Today (Jul 17, 2023), I renamed the `good first contribution` label to `good first issue`. This is because
1) `good first issue` is more commonly used for this intent across GitHub and and thus may be (ever so slightly) more recognizable to potential new contributors, and, more importantly,
2) GitHub recognizes the label `good first issue` when surfacing good first issues in specific places in GitHub[^1].

This PR updates Contributing.md to match the new name.

`grep` did not find any other references to `good first contribution` to update in this repo.

[^1]: [Source (GitHub docs)](https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/encouraging-helpful-contributions-to-your-project-with-labels). Example: https://github.com/flutter/flutter/contribute. If my vague memory serves me correctly, this can also appear in certain emails or the "discovery queue"-type experience that GitHub provides.