Update cuML install docs

[csadorf]

What does this PR do?

Updates the cuML installation examples to install BERTopic and the matching cuML package in a single command:

pip install bertopic cuml-cu12

and:

pip install bertopic cuml-cu13

This removes the previous warning that users should install cuML first because installing both packages together can fail due to pip resolver limitations. The original RAPIDS cuML issue (rapidsai/cuml#7374) that motivated that workaround no longer reproduces with the current resolver/package set.

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?