Fix typo "dependency-group" in error message and comments (#11211)
<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:
- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->
## Summary
I got a bit confused when testing `[dependency-groups]` because uv's
error message had the same typo I did in my `pyproject.toml`.
I tried to fix it, as well as a few comment I found along the way.
