[trivial] fix erronious documentation on dim

[Yurlungur]

PR Summary

Alex Long discovered that the documentation incorrectly states dim is 0-indexed when it is in fact 1-indexed. This tiny fix changes the documentation to match the code.

PR Checklist

• Code is formatted. (You can use the format_spiner make target.)
• Adds a test for any bugs fixed. Adds tests for new features.
• Document any new features, update documentation for changes made.
• Update the CHANGELOG.md file.
• Make sure the copyright notice on any files you modified is up to date.
• LANL employees: make sure tests pass both on the github CI and on the Darwin CI
• If ML was used, make sure to add a disclaimer at the top of a file indicating ML was used to assist in generating the file.
• If Agentic AI was used, have the AI generate a "proposed changes" markdown file and store it in the plan_histories folder, with a filename the same as the MR number.

If preparing for a new release, in addition please check the following:

• Update the version in cmake.
• Maintainers: ensure spackages are up to date:
  • LANL-internal team, update XCAP spackages
  • Current maintainer of upstream spackages, submit MR to spack

[alexrlongne]

Thanks for the quick fix!