Revert "rename vignette files" (b/c affects website urls!)

This reverts commit 81f2ee1.

Author: jgabry

vignettes/elpd.Rmd vignettes/loo2-elpd.Rmdvignettes/elpd.Rmd renamed to vignettes/loo2-elpd.Rmd

@@ -141,7 +141,7 @@ The `loo()` function output warnings that there are some observations which are
 highly influential, and thus the accuracy of importance sampling is compromised
 as indicated by the large Pareto $k$ diagnostic values (> 0.7). As discussed in
 the vignette
-[_Using the loo package (version >= 2.0.0)_](http://mc-stan.org/loo/articles/loo2-example.html),
+[_Using the loo package (version >= 2.0.0)_](https://mc-stan.org/loo/articles/loo2-example.html),
 this may be an indication of model misspecification. Despite that, it is still
 beneficial to be able to evaluate the predictive performance of the model
 accurately.