[MINOR][DOCS] Fix comments of SQLExecution#withExecutionId

## What changes were proposed in this pull request?
Fix comment. Change `BroadcastHashJoin.broadcastFuture` to `BroadcastExchangeExec.relationFuture`: https://github.com/apache/spark/blob/d28d5732ae205771f1f443b15b10e64dcffb5ff0/sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/BroadcastExchangeExec.scala#L66

## How was this patch tested?
N/A

Author: seancxmao <seancxmao@gmail.com>

Closes apache#21113 from seancxmao/SPARK-13136.

sql/core/src/main/scala/org/apache/spark/sql/execution/SQLExecution.scala

@@ -88,7 +88,7 @@ object SQLExecution {
   /**
    * Wrap an action with a known executionId. When running a different action in a different
    * thread from the original one, this method can be used to connect the Spark jobs in this action
-   * with the known executionId, e.g., `BroadcastHashJoin.broadcastFuture`.
+   * with the known executionId, e.g., `BroadcastExchangeExec.relationFuture`.
    */
   def withExecutionId[T](sc: SparkContext, executionId: String)(body: => T): T = {
     val oldExecutionId = sc.getLocalProperty(SQLExecution.EXECUTION_ID_KEY)