links: update slack link in FAQ to invite

Previously, it was liking directly to `firecracker-microvm.slack.com`.

Now, it's linking to the shortcut tinyurl: `https://tinyurl.com/firecracker-microvm`

Author: raduweiss

index.html

@@ -172,7 +172,7 @@
                     </article>
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                         <header>How can I contribute?</header>
-                        <section>Firecracker is an AWS open source project that encourages contributions from customers and the developer community. Any contribution is welcome as long as it aligns with our <a href="https://github.com/firecracker-microvm/firecracker/blob/master/CHARTER.md">charter</a>. You can learn more about how to contribute in <a href="https://github.com/firecracker-microvm/firecracker/blob/master/CONTRIBUTE.md">CONTRIBUTE.md</a>. You can chat with others in the community on the <a href="https://firecracker-microvm.slack.com">Firecracker Slack workspace</a>.</section>
+                        <section>Firecracker is an AWS open source project that encourages contributions from customers and the developer community. Any contribution is welcome as long as it aligns with our <a href="https://github.com/firecracker-microvm/firecracker/blob/master/CHARTER.md">charter</a>. You can learn more about how to contribute in <a href="https://github.com/firecracker-microvm/firecracker/blob/master/CONTRIBUTE.md">CONTRIBUTE.md</a>. You can chat with others in the community on the <a href="https://tinyurl.com/firecracker-microvm">Firecracker Slack workspace</a>.</section>
                     </article>            
                 </div>
                 <div class="m-banner">