[SPARK-19550][SPARKR][DOCS] Update R document to use JDK8

## What changes were proposed in this pull request?

Update R document to use JDK8.

## How was this patch tested?

manual tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #17162 from wangyum/SPARK-19550.

R/WINDOWS.md

@@ -6,7 +6,7 @@ To build SparkR on Windows, the following steps are required
 include Rtools and R in `PATH`.
 
 2. Install
-[JDK7](http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html) and set
+[JDK8](http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html) and set
 `JAVA_HOME` in the system environment variables.
 
 3. Download and install [Maven](http://maven.apache.org/download.html). Also include the `bin`