[DOCS] Fixes build issue in upgrade.asciidoc

Author: lcawl

docs/reference/release-notes/6.3.asciidoc

@@ -1,7 +1,8 @@
 [[release-notes-6.3.0]]
 == {es} version 6.3.0
 
-=== Known Issues
+[float]
+=== Known issues
 
 Clusters with a Gold or Platinum license that are upgrading to 6.3 will need to explicitly set
 `xpack.security.enabled: true` in `elasticsearch.yml` to upgrade successfully.

docs/reference/upgrade.asciidoc

@@ -1,14 +1,15 @@
 [[setup-upgrade]]
 = Upgrade Elasticsearch
 
+[partintro]
+--
 [IMPORTANT]
-
+===========================================
 Clusters with a Gold or Platinum license that are upgrading to 6.3 will need to explicitly set
 `xpack.security.enabled: true` in `elasticsearch.yml` to upgrade successfully.
 If this value is not set, the cluster will be unable to form after upgrade.
+===========================================
 
-[partintro]
---
 Elasticsearch can usually be upgraded using a <<rolling-upgrades,Rolling upgrade>>
 process so upgrading does not interrupt service. However, you might
 need to <<reindex-upgrade,Reindex to upgrade>> indices created in older versions.