docs: Update link to Disqus alternative Livefyre.com

Livefyre.com is down.

Livefyre was integrated into Adobe's offering.

The product is now called Adobe Experience Manager Livefyre.

Wikipedia points to web.livefyre.com which redirects to the link I provided in the commit.

content/extras/comments.md

@@ -74,7 +74,7 @@ A few alternatives exist to [Disqus](https://disqus.com/):
 
 * [Discourse](http://www.discourse.org)
 * [IntenseDebate](http://intensedebate.com/)
-* [Livefyre](http://livefyre.com/)
+* [Livefyre](http://www.adobe.com/marketing-cloud/enterprise-content-management/ugc-content-platform.html)
 * [Muut](http://muut.com/)
 * [多说](http://duoshuo.com/) ([Duoshuo](http://duoshuo.com/), popular in China)
 * [isso](http://posativ.org/isso/) (Self-hosted, Python)