Update Threads_CreateChatThread Example (Azure#29034)

* Update Threads_CreateChatThread Example

* update the example for create chat thread

Author: LuChen-Microsoft

specification/communication/data-plane/Chat/preview/2024-03-15-preview/examples/Threads_CreateChatThread.json

@@ -4,6 +4,10 @@
     "api-version": "2024-03-15-preview",
     "createChatThreadRequest": {
       "topic": "Lunch",
+      "retentionPolicy": {
+        "kind": "threadCreationDate",
+        "deleteThreadAfterDays": 30
+      },
       "participants": [
         {
           "communicationIdentifier": {
@@ -59,6 +63,10 @@
             "communicationUser": {
               "id": "8:acs:8540c0de-899f-5cce-acb5-3ec493af3800_c94ff260-162d-46d6-94fd-e79f4d213715"
             }
+          },
+          "retentionPolicy": {
+            "kind": "threadCreationDate",
+            "deleteThreadAfterDays": 30
           }
         },
         "invalidParticipants": [