[SPARK-19626][YARN] Using the correct config to set credentials update time

## What changes were proposed in this pull request?

In https://github.com/apache/spark/pull/14065, we introduced a configurable credential manager for Spark running on YARN. Also two configs `spark.yarn.credentials.renewalTime` and `spark.yarn.credentials.updateTime` were added, one is for the credential renewer and the other updater. But now we just query `spark.yarn.credentials.renewalTime` by mistake during CREDENTIALS UPDATING, where should be actually `spark.yarn.credentials.updateTime` .

This PR fixes this mistake.

## How was this patch tested?

existing test

cc jerryshao vanzin

Author: Kent Yao <yaooqinn@hotmail.com>

Closes #16955 from yaooqinn/cred_update.

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/security/CredentialUpdater.scala

@@ -55,7 +55,7 @@ private[spark] class CredentialUpdater(
 
   /** Start the credential updater task */
   def start(): Unit = {
-    val startTime = sparkConf.get(CREDENTIALS_RENEWAL_TIME)
+    val startTime = sparkConf.get(CREDENTIALS_UPDATE_TIME)
     val remainingTime = startTime - System.currentTimeMillis()
     if (remainingTime <= 0) {
       credentialUpdater.schedule(credentialUpdaterRunnable, 1, TimeUnit.MINUTES)