Small doc fix for ReuseSubquery.

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

@@ -150,7 +150,7 @@ case class PlanSubqueries(sparkSession: SparkSession) extends Rule[SparkPlan] {
 
 
 /**
- * Find out duplicated exchanges in the spark plan, then use the same exchange for all the
+ * Find out duplicated subqueries in the spark plan, then use the same subquery result for all the
  * references.
  */
 case class ReuseSubquery(conf: SQLConf) extends Rule[SparkPlan] {
@@ -159,7 +159,7 @@ case class ReuseSubquery(conf: SQLConf) extends Rule[SparkPlan] {
     if (!conf.exchangeReuseEnabled) {
       return plan
     }
-    // Build a hash map using schema of exchanges to avoid O(N*N) sameResult calls.
+    // Build a hash map using schema of subqueries to avoid O(N*N) sameResult calls.
     val subqueries = mutable.HashMap[StructType, ArrayBuffer[SubqueryExec]]()
     plan transformAllExpressions {
       case sub: ExecSubqueryExpression =>