Update site_config.mdx

Author: mibali

docs/admin/config/site_config.mdx

@@ -220,7 +220,7 @@ All site configuration options and their default values are shown below.
 	//     "notifier": {
 	//       "channel": "#alerts",
 	//       "type": "slack",
-	//       "url": "https://hooks.slack.com/s/..."
+	//       "url": "https://hooks.slack.com/services/..."
 	//     }
 	//   }
 	// - {