[MINOR][DOC] Add missing call of update() in examples of PeriodicGraphCheckpointer & PeriodicRDDCheckpointer

## What changes were proposed in this pull request?
forgot to call `update()` with `graph1` & `rdd1` in examples for `PeriodicGraphCheckpointer` & `PeriodicRDDCheckpoin`
## How was this patch tested?
existing tests

Author: Zheng RuiFeng <[email protected]>

Closes apache#19198 from zhengruifeng/fix_doc_checkpointer.

Author: zhengruifeng

core/src/main/scala/org/apache/spark/rdd/util/PeriodicRDDCheckpointer.scala

@@ -50,6 +50,7 @@ import org.apache.spark.util.PeriodicCheckpointer
  * {{{
  *  val (rdd1, rdd2, rdd3, ...) = ...
  *  val cp = new PeriodicRDDCheckpointer(2, sc)
+ *  cp.update(rdd1)
  *  rdd1.count();
  *  // persisted: rdd1
  *  cp.update(rdd2)

graphx/src/main/scala/org/apache/spark/graphx/util/PeriodicGraphCheckpointer.scala

@@ -50,6 +50,7 @@ import org.apache.spark.util.PeriodicCheckpointer
  * {{{
  *  val (graph1, graph2, graph3, ...) = ...
  *  val cp = new PeriodicGraphCheckpointer(2, sc)
+ *  cp.updateGraph(graph1)
  *  graph1.vertices.count(); graph1.edges.count()
  *  // persisted: graph1
  *  cp.updateGraph(graph2)