update PSIS references in vignettes

[jgabry]

@avehtari This just updates the vignettes to also use the new citation for the PSIS paper. And I guess we'll need to update everything again once it's actually published.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.76%. Comparing base (ccd51a7) to head (0a9d5f2).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #254   +/-   ##
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  Coverage   92.76%   92.76%           
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  Files          31       31           
  Lines        2834     2834           
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  Hits         2629     2629           
  Misses        205      205           

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