Fix incorrect parameter name in cumulative() docstrings

[Sreekant13]

Description

The docstrings for DataArray.cumulative and Dataset.cumulative document the parameter as dims, but the actual signature parameter is dim. As a result, calling obj.cumulative(dims=...) raises a TypeError even though the docs show
dims. This corrects dims → dim in both Parameters sections to match the signature. Documentation-only change; no behavior change.

Checklist

• Closes #xxxx
• Tests added
• User visible changes documented in whats-new.rst
• New functions/methods listed in api.rst

(Docstring-only correction — no tests, whats-new, or api.rst entry needed.)

AI Disclosure

• This PR contains AI-generated content.
• I have tested any AI-generated content in my PR.
• I take responsibility for any AI-generated content in my PR.
  Tools: Claude (Claude Code) was used to help identify the docstring/signature mismatch. I reviewed the change against the cumulative signatures in both dataarray.py and dataset.py, confirmed dim is the correct parameter name, and take full responsibility for the change.

[Sreekant13]

Heads-up: the failing docs/readthedocs.org:xray check looks unrelated to this change. The build fails during environment setup at the pixi install -e doc step (before any docs are rendered), and the same docs/readthedocs.org:xray
status is currently failing on all the other open PRs I checked. So it appears to be a repo-wide docs-build issue rather than anything in this PR.

This is a documentation-only change (dims → dim in two docstrings); happy to rebase or re-run once the docs build is healthy again. Thanks!