Refer to external commit tracking as "illumos" rather than "OpenZFS"

[ahrens]

The main docs page (https://openzfs.github.io/openzfs-docs/) talks about "OpenZFS tracking/patches/exceptions". Since this repo is now OpenZFS, I think these should refer to "illumos" rather than "OpenZFS".

(FYI, I think the file to edit is https://github.com/openzfs/openzfs-docs/blob/master/docs/Developer%20Resources/index.rst )