Update I/O banner for tuning in

Author: parlough

src/_includes/banner.html

@@ -1,6 +1,6 @@
 <div class="banner">
   <p class="banner__text">
-    Dart and Flutter are back at Google I/O on May 14!
-    <a href="https://io.google/2024/?utm_source=flutter&utm_medium=embedded_marketing" target="_blank">Register now</a>
+    Dart and Flutter are back at Google I/O!
+    <a href="https://io.google/2024/?utm_source=flutter&utm_medium=embedded_marketing" target="_blank">Tune in now</a>
   </p>
 </div>