Fix another old reference to optimization levels

[gbrail]

Fix a spot I missed when updating the README.

Thanks @andreabergia!

[p-bakker]

Sry I'm not able to pitch in a PR, but there are more references to optimization levels in our docs, like https://rhino.github.io/docs/configuration/#optimization-levels (see https://github.com/rhino/rhino.github.io/blob/main/docs/_docs/configuration.md)

[gbrail]

Yes, the docs in the other repo need an update, and that will probably come some later time, like in January...