[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)(_ + _)

Author: Yadong Qi <qiyadong2010@gmail.com>

Closes #2182 from watermen/bug-fix3 and squashes the following commits:

17be9fb [Yadong Qi] Update CheckpointSuite.scala
714bda5 [Yadong Qi] Update BasicOperationsSuite.scala
57e704c [Yadong Qi] Update StatefulNetworkWordCount.scala