Replace wrongly formatted dummy Slack URL with properly formatted dummy Slack URL

Signed-off-by: Aniruddh <aniruddh@purdue.edu>

Author: Noir01

notifications/notifications/src/test/kotlin/org/opensearch/notifications/model/NotificationConfigDocTests.kt

@@ -67,7 +67,7 @@ internal class NotificationConfigDocTests {
                 "description":"description",
                 "config_type":"slack",
                 "is_enabled":true,
-                "slack":{"url":"https://domain.com/sample_url#1234567890"}
+                "slack":{"url":"https://hooks.slack.com/services/sample_slack_url"}
             },
             "extra_field_1":["extra", "value"],
             "extra_field_2":{"extra":"value"},