Update vuemastery promotion copy (#1007)

Author: pieer

src/.vuepress/theme/components/sponsors/VueMasteryBanner.vue

@@ -4,7 +4,7 @@
     <div class="vuemastery-banner--wrapper">
       <img class="vuemastery-banner--logo" src="/images/vuemastery/logo-vuemastery.svg" alt="vuemastery" />
       <p><span>Watch all Vue Mastery courses</span>Free all weekend on April 23-25</p>
-      <button class="vuemastery-button">Reserve your spot</button>
+      <button class="vuemastery-button">Register now</button>
     </div>
     <div id="vm-close" class="vuemastery-banner--close" @click.prevent="$emit('close-banner')"></div>
   </a>