Update R document to use JDK8

Author: wangyum

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`