[SPARK-26394][CORE] Fix annotation error for Utils.timeStringAsMs

## What changes were proposed in this pull request?

Change microseconds to milliseconds in annotation of Utils.timeStringAsMs.

Closes apache#23346 from stczwd/stczwd.

Authored-by: Jackey Lee <qcsd2011@163.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

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

@@ -1037,7 +1037,7 @@ private[spark] object Utils extends Logging {
   }
 
   /**
-   * Convert a time parameter such as (50s, 100ms, or 250us) to microseconds for internal use. If
+   * Convert a time parameter such as (50s, 100ms, or 250us) to milliseconds for internal use. If
    * no suffix is provided, the passed number is assumed to be in ms.
    */
   def timeStringAsMs(str: String): Long = {