Corrected HTML errors in the slack links

Author: Manas Marthi

_includes/important-links.html

@@ -2,7 +2,7 @@
   <h3 class="widget-title">Join the Community</h3>
   <ul>
     <li><a class="spec" href="irc://irc.freenode.net/elixir-lang">#elixir-lang on freenode IRC</a></li>
-    <li><a class="spec" href="https://elixir-slackin.herokuapp.com/">Get Invitation to Elixir's Slack channel </a> (then visit <a class="spec" href="https://elixir-lang.slack.com/"></li>
+    <li><a class="spec" href="https://elixir-slackin.herokuapp.com/">Get Invitation to Elixir's Slack channel </a> (then visit <a class="spec" href="https://elixir-lang.slack.com/">Elixir slack</a> Site)</li>
     <li><a class="spec" href="http://elixirforum.com">Elixir Forum</a></li>
     <li><a class="spec" href="https://groups.google.com/group/elixir-lang-talk">elixir-talk mailing list</a></li>
     <li><a class="spec" href="https://twitter.com/elixirlang">@elixirlang on Twitter</a></li>