doc fix of HiveThriftServer

## What changes were proposed in this pull request?

Just minor doc fix.

\cc yhuai

Author: Jeff Zhang <zjffdu@apache.org>

Closes #13659 from zjffdu/doc_fix.

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/ui/ThriftServerPage.scala

@@ -30,7 +30,7 @@ import org.apache.spark.ui._
 import org.apache.spark.ui.UIUtils._
 
 
-/** Page for Spark Web UI that shows statistics of a thrift server */
+/** Page for Spark Web UI that shows statistics of the thrift server */
 private[ui] class ThriftServerPage(parent: ThriftServerTab) extends WebUIPage("") with Logging {
 
   private val listener = parent.listener

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/ui/ThriftServerSessionPage.scala

@@ -29,7 +29,7 @@ import org.apache.spark.sql.hive.thriftserver.HiveThriftServer2.{ExecutionInfo,
 import org.apache.spark.ui._
 import org.apache.spark.ui.UIUtils._
 
-/** Page for Spark Web UI that shows statistics of a streaming job */
+/** Page for Spark Web UI that shows statistics of jobs running in the thrift server */
 private[ui] class ThriftServerSessionPage(parent: ThriftServerTab)
   extends WebUIPage("session") with Logging {
 
@@ -60,7 +60,7 @@ private[ui] class ThriftServerSessionPage(parent: ThriftServerTab)
     UIUtils.headerSparkPage("JDBC/ODBC Session", content, parent, Some(5000))
   }
 
-  /** Generate basic stats of the streaming program */
+  /** Generate basic stats of the thrift server program */
   private def generateBasicStats(): Seq[Node] = {
     val timeSinceStart = System.currentTimeMillis() - startTime.getTime
     <ul class ="unstyled">

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/ui/ThriftServerTab.scala

@@ -24,7 +24,7 @@ import org.apache.spark.sql.hive.thriftserver.ui.ThriftServerTab._
 import org.apache.spark.ui.{SparkUI, SparkUITab}
 
 /**
- * Spark Web UI tab that shows statistics of a streaming job.
+ * Spark Web UI tab that shows statistics of jobs running in the thrift server.
  * This assumes the given SparkContext has enabled its SparkUI.
  */
 private[thriftserver] class ThriftServerTab(sparkContext: SparkContext)