[MINOR][R] Deduplicate RStudio setup documentation

## What changes were proposed in this pull request?

This PR targets to deduplicate RStudio setup for SparkR.

## How was this patch tested?

N/A

Closes #23421 from HyukjinKwon/minor-doc.

Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

R/README.md

@@ -39,15 +39,7 @@ To set other options like driver memory, executor memory etc. you can pass in th
 
 #### Using SparkR from RStudio
 
-If you wish to use SparkR from RStudio or other R frontends you will need to set some environment variables which point SparkR to your Spark installation. For example
-```R
-# Set this to where Spark is installed
-Sys.setenv(SPARK_HOME="/Users/username/spark")
-# This line loads SparkR from the installed directory
-.libPaths(c(file.path(Sys.getenv("SPARK_HOME"), "R", "lib"), .libPaths()))
-library(SparkR)
-sparkR.session()
-```
+If you wish to use SparkR from RStudio, please refer [SparkR documentation](https://spark.apache.org/docs/latest/sparkr.html#starting-up-from-rstudio).
 
 #### Making changes to SparkR