[DOCS] Fix broken Cloud link for 5.6 (#301)

Updated the `ec-upgrade-cluster.html` link to `ec-upgrade-deployment.html`.

Author: KOTungseth

docs/en/install-upgrade/upgrading-stack.asciidoc

@@ -142,5 +142,5 @@ To avoid downtime on production clusters due to major version upgrades:
 
 . Delete the old cluster to stop incurring additional costs. You are billed extra only for the time that the additional cluster was running. Billing for usage is by the hour.
 
-To learn more about the upgrade process on Elastic Cloud, see {cloudref}/ec-upgrade-cluster.html[
+To learn more about the upgrade process on Elastic Cloud, see {cloudref}/ec-upgrade-deployment.html[
 Upgrade Versions] and {cloudref}/ec-configure.html[Configuring Elastic Cloud].