fix: update structured concurrency example to Java 26 Joiner API

Signed-off-by: https://github.com/Someshdiwan <someshdiwan369@gmail.com>

Author: Someshdiwan

Java-Versions/Java 26/src/StructuredConcurrencyDemo.java

@@ -32,8 +32,6 @@ static String fetchData(String name) throws InterruptedException {
 StructuredTaskScope API changed from older ShutdownOnFailure style
 to the new open(...) + Joiner style.
 
-
-
 Previous Java style:
 - Usually we used ExecutorService + Future
 - We manually submitted tasks