[SPARK-16214][EXAMPLES] fix the denominator of SparkPi

## What changes were proposed in this pull request?

reduce the denominator of SparkPi by 1

## How was this patch tested?

  integration tests

Author: 杨浩 <yanghaogn@163.com>

Closes #13910 from yanghaogn/patch-1.

examples/src/main/scala/org/apache/spark/examples/SparkPi.scala

@@ -36,7 +36,7 @@ object SparkPi {
       val y = random * 2 - 1
       if (x*x + y*y < 1) 1 else 0
     }.reduce(_ + _)
-    println("Pi is roughly " + 4.0 * count / n)
+    println("Pi is roughly " + 4.0 * count / (n - 1))
     spark.stop()
   }
 }