Fix Slack link

Closes #8

Author: Brandon Johnson

pages/index.html

@@ -19,7 +19,7 @@
         <span class="icon rsvp"></span>
         <span>RSVP for Event</span>
       </button>
-      <button class="outline-button" onclick="window.open(https://javascript-mn.slack.com)">
+      <button class="outline-button" onclick="window.open('https://javascriptmn-slack.herokuapp.com')">
         <span class="icon slack"></span>
         <span>Join our Slack</span>
       </button>