Add 'ofSet' to .codespellignore (See #1176)

[Bergschaf]

No description provided.

[github-actions]

Thank you for this PR, which will now be reviewed.
If submitting to ./Physlib or ./QuantumInfo, please
see our review guidelines
if you are not familiar with the process. You should expect a back and forth
with a reviewer before your PR is merged. See also that link for how to
add appropriate labels to your PR. The PR will also go through a number
of automated checks. You can learn more about these here,
including how to run them locally.

If you are submitting to ./PhyslibAlpha there will be a lighter review process,
though your PR must still pass the automated checks.

If you want to bring attention to this PR, please write a message on this
thread of the Lean Zulip.

[jstoobysmith]

Approved - many thanks - will merge when ready.

[Bergschaf]

That was fast, thank you!