[MINOR][SS] Remove duplicate 'add' in comment of `StructuredSessioniz…

…ation`.

## What changes were proposed in this pull request?

`StructuredSessionization` comment contains duplicate 'add', I think it should be changed.

## How was this patch tested?

Exists UT.

Closes #24589 from beliefer/remove-duplicate-add-in-comment.

Lead-authored-by: gengjiaan <gengjiaan@360.cn>
Co-authored-by: Jiaan Geng <beliefer@163.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
(cherry picked from commit 7dd2dd5)
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

examples/src/main/scala/org/apache/spark/examples/sql/streaming/StructuredSessionization.scala

@@ -70,7 +70,7 @@ object StructuredSessionization {
         line.split(" ").map(word => Event(sessionId = word, timestamp))
       }
 
-    // Sessionize the events. Track number of events, start and end timestamps of session, and
+    // Sessionize the events. Track number of events, start and end timestamps of session,
     // and report session updates.
     val sessionUpdates = events
       .groupByKey(event => event.sessionId)