chore: fix button playground example to use prefix slot (#10321)

Author: jouni

dev/playground/button.html

@@ -83,7 +83,7 @@ <h2 class="heading">Custom Layout</h2>
         Reply
       </vaadin-button>
       <vaadin-button style="flex-direction: column">
-        <vaadin-icon icon="vaadin:reply"></vaadin-icon>
+        <vaadin-icon icon="vaadin:reply" slot="prefix"></vaadin-icon>
         Reply
       </vaadin-button>
     </section>