docs: Minor README fixes

[jwm4]

Some small tweaks here:

1. The README was referring to the project ID for watsonx.ai as an "instance ID" which seemed confusing so I changed it to project ID.
2. The README omitted the URL for watsonx.ai. Maybe that's fine because lots of people use US South but some people don't use US South so I think it's nicer to just make it explicit that this is an optional parameter that defaults to the US South URL.
3. Made the arguments to the generate_from_sample and critique a Path instead of a string. I think it would be nice if these methods took a string, but when I tried it, it seems like it only takes a Path

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[ceberam]

• Thanks @jwm4 for your interest and contribution to docling-sdg.
  I agree with your comments and I will take your suggestion of allowing a string, in addition to Path in a next release.
• By the way, since you suggested replacing WATSONX_INSTANCE_ID with WATSONX_PROJECT_ID, could you also apply this change in the command line app and in the environment file example?
  https://github.com/docling-project/docling-sdg/blob/main/docling_sdg/cli/qa.py#L299
  https://github.com/docling-project/docling-sdg/blob/main/docling_sdg/cli/qa.py#L383
  https://github.com/docling-project/docling-sdg/blob/main/docling_sdg/resources/watsonx_example.env#L3
• Also, please use docs: in the PR message instead of fix: since the changes in README are related to documentation

[jwm4]

Thanks! I have made these changes.

[ceberam]

Thanks @jwm4 for your contribution.
For next PRs, please consider rebasing your branch to main instead of creating a merge commit to keep the git log simpler.