This is how we would advise you to reference PRISMA. You will just need to change the cited date to the date that you viewed it online.
Note: Cited dates must always follow the formula of “Year Month Day”. Always abbreviate the month to the first 3 letters.
Journal name [Internet].
Year of publication [cited date];
Inclusive page number(s) of the article.
Moher D, Liberati A, Tetzlaff J, Altman DG. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. PLoS Medicine [Internet]. 2009 [cited 2018 Feb 9]; 6(7): e1000097 [5p.]. Available from: https://doi.org/10.1371/journal.pmed.1000097