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:

Book:

https://link.springer.com/chapter/10.1007/978-3-319-32370-1_9

Journal papers:

Agirre-Olabide et al 2017 Smart Mater. Struct. 26 035021

Kang et al 2015 Trans Kor Soc Mech Eng 39(11) 1183-1189

Chugh 2013 Num and Anal Meth in Geomech

Conference:

Mardalizad et al 2015 10th Euro LS-DYNA Conf

Kulkarni et al 2015 ISSS National conf on MEMS Smart Materials

Thesis:

van Rijbroek 2015 Eindhoven UoT

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 …

http://www.ipl.org/div/farq/netciteFARQ.html