Update header.ejs

Author: rigor789

layouts/_partials/header.ejs

@@ -1,11 +1,9 @@
-<!--
-<a href="https://attendee.gotowebinar.com/register/9059771692589680643?source=nativescript-vue"
+<a href="https://battle.tighten.co/?source=nativescript-vue.org"
    target="_blank"
    rel="noopener"
    class="block bg-blue-light md:bg-green md:border-t-8 border-green p-4 md:p-0 text-center no-underline text-white">
-    Sign up for "A New Vue for NativeScript" &mdash; A free Webinar in April 2018
+   May 25: Tighten Dev Battle 2: Four Brains. Two Screens. One Native App.
 </a>
--->
 <a href="/blog/nativescript-vue-1.0-and-a-new-site/"
    class="block md:hidden bg-green p-2 text-center no-underline text-white">
     Announcing 1.0