chore: change slack -> discord

Author: rigor789

layouts/_partials/header.ejs

@@ -91,10 +91,10 @@
                            target="_blank"
                            rel="noopener"
                            class="no-underline text-blue-lightest md:text-blue-dark hover:bg-green hover:text-white px-4 py-2">GitHub</a>
-                        <a href="https://www.nativescript.org/slack-invitation-form"
+                        <a href="https://www.nativescript.org/discord"
                            target="_blank"
                            rel="noopener"
-                           class="no-underline text-blue-lightest md:text-blue-dark hover:bg-green hover:text-white px-4 py-2">Slack</a>
+                           class="no-underline text-blue-lightest md:text-blue-dark hover:bg-green hover:text-white px-4 py-2">Discord</a>
                         <a href="https://twitter.com/VueNativeScript"
                            target="_blank"
                            rel="noopener"