[SPARK-29182][CORE][FOLLOWUP] Cache preferred locations of checkpointed RDD

### What changes were proposed in this pull request?

This is a followup to apache#25856. This fixes the document about the config value of spark.rdd.checkpoint.cachePreferredLocsExpireTime.

### Why are the changes needed?

The document is not correct. spark.rdd.checkpoint.cachePreferredLocsExpireTime can not be 0.

### Does this PR introduce any user-facing change?

No

### How was this patch tested?

This is document only change.

Closes apache#26251 from viirya/SPARK-29182-followup.

Authored-by: Liang-Chi Hsieh <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: viirya

core/src/main/scala/org/apache/spark/internal/config/package.scala

@@ -804,7 +804,7 @@ package object config {
       .doc("Expire time in minutes for caching preferred locations of checkpointed RDD." +
         "Caching preferred locations can relieve query loading to DFS and save the query " +
         "time. The drawback is that the cached locations can be possibly outdated and " +
-        "lose data locality. If this config is not specified or is 0, it will not cache.")
+        "lose data locality. If this config is not specified, it will not cache.")
       .timeConf(TimeUnit.MINUTES)
       .checkValue(_ > 0, "The expire time for caching preferred locations cannot be non-positive.")
       .createOptional