Fix unbalanced knitr chunk delimiters

[jgabry]

Notification from Yihui, the knitr maintainer:

Two years ago I announced an upcoming breaking change in knitr: https://yihui.org/en/2021/10/unbalanced-delimiters/ This will affect your loo package on CRAN. I wonder if you could remove the extra backticks of code chunks in your package like this: https://github.com/stan-dev/loo/blob/master/vignettes/loo2-non-factorized.Rmd#L572

You can test by remotes::install_github('yihui/knitr#2306') and rebuilding the package to see if there are any other unbalanced code chunks. Thank you!