Fix thread safety issue in connection pool example

Co-authored-by: janbodnar <16985640+janbodnar@users.noreply.github.com>

Author: Copilot

virtualthreads-structuredconcurrency.md

@@ -1935,15 +1935,15 @@ void main() throws Exception {
 
 class ConnectionPool {
     private final Semaphore semaphore;
-    private int connectionId = 0;
+    private final java.util.concurrent.atomic.AtomicInteger connectionId = new java.util.concurrent.atomic.AtomicInteger(0);
 
     ConnectionPool(int size) {
         this.semaphore = new Semaphore(size);
     }
 
     Connection acquire() throws InterruptedException {
         semaphore.acquire();
-        return new Connection(++connectionId, this);
+        return new Connection(connectionId.incrementAndGet(), this);
     }
 
     void release() {