update slack invite link

Signed-off-by: Alexandra Iordache <[email protected]>

Author: Alexandra Iordache

index.html

@@ -85,7 +85,7 @@
                         <i class="icon-github" aria-hidden="true"></i>
                         <span>join our github community</span>
                     </a>
-                    <a href="https://tinyurl.com/firecracker-microvm" class="css-button sta-hoverOFF">
+                    <a href="https://join.slack.com/t/firecracker-microvm/shared_invite/enQtNDY2NTUwMzQ3MDE1LWIwMzA0OWFkMTZhMTlmMDZiMmFkYjMyODMxMGQ1ZjliMzJjNjJiNWRhNWNkOGEyNmUxNmRkMjZhYTc3MmVjZjM" class="css-button sta-hoverOFF">
                         <i class="icon-slack" aria-hidden="true"></i>
                         <span>Chat about Firecracker on Slack</span>
                     </a>