There’s no “correct way” to reference on-line notes/books.
The year you refer to would be the year it is now.
Here are examples of how others have referred to these notes:
A simple version would be something like:
Kelly, PA. Mechanics Lecture Notes: An introduction to Solid Mechanics. Available from http://homepages.engineering.auckland.ac.nz/~pkel015/SolidMechanicsBooks/index.html
If you really want to delve into the intricacies of this topic …