Replace reference to ui-samples-plugin with design-library in javadoc

ui-samples-plugin is suspended and has been replaced with design-library:

> Plugin distribution has been suspended, see https://groups.google.com/g/jenkinsci-dev/c/2vGn3t9gZ0Y for details.
https://plugins.jenkins.io/ui-samples-plugin/

jenkinsci/design-library-plugin#14

core/src/main/java/jenkins/util/ProgressiveRendering.java

@@ -70,7 +70,7 @@
  * <li>As results become available, call {@link #progress}.
  * <li>Make {@link #data} produce whatever JSON you want to send to the page to be displayed.
  * </ol>
- * {@code ui-samples-plugin} demonstrates all this.
+ * {@code design-library} demonstrates all this.
  * @since 1.484
  */
 public abstract class ProgressiveRendering {