[MINOR][DOCS] Fix R documentation generation instruction for roxygen2

## What changes were proposed in this pull request?

This PR proposes to fix `roxygen2` to `5.0.1` in `docs/README.md` for SparkR documentation generation.

If I use higher version and creates the doc, it shows the diff below. Not a big deal but it bothered me.

```diff
diff --git a/R/pkg/DESCRIPTION b/R/pkg/DESCRIPTION
index 855eb5bf77f..159fca61e06 100644
--- a/R/pkg/DESCRIPTION
+++ b/R/pkg/DESCRIPTION
 -57,6 +57,6  Collate:
     'types.R'
     'utils.R'
     'window.R'
-RoxygenNote: 5.0.1
+RoxygenNote: 6.0.1
 VignetteBuilder: knitr
 NeedsCompilation: no
```

## How was this patch tested?

Manually tested. I met this every time I set the new environment for Spark dev but I have kept forgetting to fix it.

Author: hyukjinkwon <[email protected]>

Closes apache#21020 from HyukjinKwon/minor-r-doc.

docs/README.md

@@ -22,10 +22,13 @@ $ sudo gem install jekyll jekyll-redirect-from pygments.rb
 $ sudo pip install Pygments
 # Following is needed only for generating API docs
 $ sudo pip install sphinx pypandoc mkdocs
-$ sudo Rscript -e 'install.packages(c("knitr", "devtools", "roxygen2", "testthat", "rmarkdown"), repos="http://cran.stat.ucla.edu/")'
+$ sudo Rscript -e 'install.packages(c("knitr", "devtools", "testthat", "rmarkdown"), repos="http://cran.stat.ucla.edu/")'
+$ sudo Rscript -e 'devtools::install_version("roxygen2", version = "5.0.1", repos="http://cran.stat.ucla.edu/")'
 ```
 
-(Note: If you are on a system with both Ruby 1.9 and Ruby 2.0 you may need to replace gem with gem2.0)
+Note: If you are on a system with both Ruby 1.9 and Ruby 2.0 you may need to replace gem with gem2.0.
+
+Note: Other versions of roxygen2 might work in SparkR documentation generation but `RoxygenNote` field in `$SPARK_HOME/R/pkg/DESCRIPTION` is 5.0.1, which is updated if the version is mismatched.
 
 ## Generating the Documentation HTML
 
@@ -62,12 +65,12 @@ $ PRODUCTION=1 jekyll build
 
 ## API Docs (Scaladoc, Javadoc, Sphinx, roxygen2, MkDocs)
 
-You can build just the Spark scaladoc and javadoc by running `build/sbt unidoc` from the `SPARK_HOME` directory.
+You can build just the Spark scaladoc and javadoc by running `build/sbt unidoc` from the `$SPARK_HOME` directory.
 
 Similarly, you can build just the PySpark docs by running `make html` from the
-`SPARK_HOME/python/docs` directory. Documentation is only generated for classes that are listed as
-public in `__init__.py`. The SparkR docs can be built by running `SPARK_HOME/R/create-docs.sh`, and
-the SQL docs can be built by running `SPARK_HOME/sql/create-docs.sh`
+`$SPARK_HOME/python/docs` directory. Documentation is only generated for classes that are listed as
+public in `__init__.py`. The SparkR docs can be built by running `$SPARK_HOME/R/create-docs.sh`, and
+the SQL docs can be built by running `$SPARK_HOME/sql/create-docs.sh`
 after [building Spark](https://github.com/apache/spark#building-spark) first.
 
 When you run `jekyll build` in the `docs` directory, it will also copy over the scaladoc and javadoc for the various