[SPARK-23708][CORE] Correct comment for function addShutDownHook in S…

…hutdownHookManager

## What changes were proposed in this pull request?
Minor modification.Comment below is not right.
```
/**
   * Adds a shutdown hook with the given priority. Hooks with lower priority values run
   * first.
   *
   * param hook The code to run during shutdown.
   * return A handle that can be used to unregister the shutdown hook.
   */
  def addShutdownHook(priority: Int)(hook: () => Unit): AnyRef = {
    shutdownHooks.add(priority, hook)
  }
```

## How was this patch tested?

UT

Author: zhoukang <[email protected]>

Closes apache#20845 from caneGuy/zhoukang/fix-shutdowncomment.

core/src/main/scala/org/apache/spark/util/ShutdownHookManager.scala

@@ -143,7 +143,7 @@ private[spark] object ShutdownHookManager extends Logging {
   }
 
   /**
-   * Adds a shutdown hook with the given priority. Hooks with lower priority values run
+   * Adds a shutdown hook with the given priority. Hooks with higher priority values run
    * first.
    *
    * @param hook The code to run during shutdown.