As is mentioned in this new post to the CodeIgniter project's main site, their wiki has made a move. You can now find it on github.

In addition to having the convenience of existing as a git repo itself, moving this content to GitHub will help distinguish EllisLab and Reactor created content from community managed resources. It will also reduce the attraction of spammers to the CodeIgniter.com web site, as the wiki was a common location used by spammers and vandals to get content on our site that was less likely to be found and moderated than the frequently visited forums.

You can make changes if you see something incorrect by cloning the repo and making changes to the wiki repo and putting in a pull request.