Update repo links to r-universe (#988)

Author: andrjohns

README.md

@@ -38,7 +38,7 @@ You can install the latest beta release of the **cmdstanr** R package with
 
 ```r
 # we recommend running this in a fresh R session or restarting your current session
-install.packages("cmdstanr", repos = c("https://mc-stan.org/r-packages/", getOption("repos")))
+install.packages("cmdstanr", repos = c('https://stan-dev.r-universe.dev', getOption("repos")))
 ```
 This does not install the vignettes, which take a long time to build, but they are always available
 online at https://mc-stan.org/cmdstanr/articles/.

vignettes/cmdstanr.Rmd

@@ -30,7 +30,7 @@ by running the following command in R.
 
 ```{r install, eval=FALSE}
 # we recommend running this is a fresh R session or restarting your current session
-install.packages("cmdstanr", repos = c("https://mc-stan.org/r-packages/", getOption("repos")))
+install.packages("cmdstanr", repos = c('https://stan-dev.r-universe.dev', getOption("repos")))
 ```
 
 We can now load the package like any other R package. We'll also load the