[SPARK-3285] [examples] Using values.sum is easier to understand than using values.foldLeft(0)(_ + _)

def sum[B >: A](implicit num: Numeric[B]): B = foldLeft(num.zero)(num.plus)
Using values.sum is easier to understand than using values.foldLeft(0)(_ + _), so we'd better use values.sum instead of values.foldLeft(0)(_ + _)