Fix a typo (#314)

Author: cuixueqin

02-set-up.Rmd

@@ -279,7 +279,7 @@ R --vanilla
 
 Two files are read each time R starts (unless one of the command line options outlined above is used):
 
-- `.Renviron`, the primary purpose of which is to set *environment variables*. These tell R where to find external programs and can hold user-specific information than needs to be kept secret, typically *API keys*. 
+- `.Renviron`, the primary purpose of which is to set *environment variables*. These tell R where to find external programs and can hold user-specific information that needs to be kept secret, typically *API keys*. 
 
 - `.Rprofile` is a plain text file (which is always called `.Rprofile`, hence its name) that simply runs lines of R code every time R starts. If you want R to check for package updates each time it starts (as explained in the previous section), you simply add the relevant line somewhere in this file.