index.html: update stale slack invitation links

The link was renewed in PR #26 , but it still has stale links.
This commit updates them.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

index.html

@@ -358,7 +358,7 @@
               </a>
               . You can chat with others in the community on the
               <a
-                href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-oxbm7tqt-GLlze9zZ7sdRSDY6OnXXHg"
+                href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-1fecwrorm-x5URTlOzBR2fExTU2mWfug"
               >
                 Firecracker Slack workspace
               </a>
@@ -421,7 +421,7 @@
             <span>Join our GitHub Community</span>
           </a>
           <a
-            href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-oxbm7tqt-GLlze9zZ7sdRSDY6OnXXHg"
+            href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-1fecwrorm-x5URTlOzBR2fExTU2mWfug"
             class="css-button3 sta-hoverOFF"
           >
             <i class="icon-slack" aria-hidden="true"></i>