fix typo in readme

Author: jgabry

README.md

@@ -37,7 +37,7 @@ analysis.
 You can install the latest beta release of the **cmdstanr** R package with
 
 ```r
-# we recommend running this is a fresh R session or restarting your current session
+# 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")))
 ```
 This does not install the vignettes, which take a long time to build, but they are always available