Purdue OWL has recently updated their URL from https://owl.english.purdue.edu/ to the new https://owl.purdue.edu/ (and more specifically the citation help page: https://owl.purdue.edu/owl/purdue_owl.html.
The website indicates that a redirect is in place, so never fear if you don't update your bookmarks!
Their press release:
It's Here: A new look for the Purdue OWL!
The new version of the Purdue OWL is available at https://owl.purdue.edu/. Worry not! Our navigation menu and content will remain largely the same. In 10 days, we will be discontinuing owl.english.purdue.edu and you will be automatically redirected to the new site.
A peek at the new website: