SPARK-2400 : fix spark.yarn.max.executor.failures explaination

According to 
```scala
  private val maxNumExecutorFailures = sparkConf.getInt("spark.yarn.max.executor.failures",
    sparkConf.getInt("spark.yarn.max.worker.failures", math.max(args.numExecutors * 2, 3)))
```
default value should be numExecutors * 2, with minimum of 3,  and it's same to the config 
`spark.yarn.max.worker.failures`