update psis paper reference

[avehtari]

psis paper has been accepted for publication

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if aa28262 is merged into master:

•   :rocket:as_draws_array: 148ms -> 147ms [-1.34%, -0.07%]
• ❗🐌as_draws_df: 74.2ms -> 76.1ms [+1.07%, +3.8%]
•   :ballot_box_with_check:as_draws_list: 175ms -> 177ms [-1.14%, +2.38%]
•   :ballot_box_with_check:as_draws_matrix: 66.9ms -> 66.8ms [-1.21%, +0.85%]
•   :rocket:as_draws_rvars: 86.7ms -> 85.7ms [-2.16%, -0.05%]
•   :ballot_box_with_check:summarise_draws_100_variables: 720ms -> 720ms [-0.9%, +0.94%]
•   :ballot_box_with_check:summarise_draws_10_variables: 120ms -> 120ms [-2.01%, +1.48%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[paul-buerkner]

Congratulations! I almost cannot believe they eventually accepted this paper after having it in review for about 6(?) years :-D