fix up Slack invite link

The URL is only set up to redirect and does not handle HTTPS.
Sorry, my bad!

Signed-off-by: 지지 ᚠד (Jiji Freya Daniel) Maslowski <info@orangecms.org>

Author: orangecms

content/_index.md

@@ -63,7 +63,7 @@ to your favorite calendar app.
 
 * [Mailing list](https://groups.google.com/forum/#!forum/linuxboot)
 * [Matrix (chat)](https://matrix.to/#/#osfw-linuxboot:matrix.org)
-* [Slack (chat)](https://osfw.slack.com/messages/linuxboot) (Join [here](https://slack.osfw.foundation/))
+* [Slack (chat)](https://osfw.slack.com/messages/linuxboot) (Join [here](http://slack.osfw.foundation/))
 * [Twitter](https://twitter.com/LinuxBootOrg)
 
 **NOTE**: Matrix and Slack are not bridged, but we are present in both.