Fix doc (not using shell build/mvn)

R/WINDOWS.md

@@ -17,9 +17,11 @@ directory in Maven in `PATH`.
 5. Open a command shell (`cmd`) in the Spark directory and build Spark with [Maven](http://spark.apache.org/docs/latest/building-spark.html#building-with-buildmvn) and include the `-Psparkr` profile to build the R package. For example to use the default Hadoop versions you can run
 
     ```bash
-    build/mvn -DskipTests -Psparkr package
+    mvn -DskipTests -Psparkr package
     ```
 
+    `build/mvn` is a shell script so `mvn` should be used directly on Windows.
+
 ##  Unit tests
 
 To run the SparkR unit tests on Windows, the following steps are required —assuming you are in the Spark root directory and do not have Apache Hadoop installed already: