[DOCS] Adds missing anchor in Installation Upgrade Guide (#223)

Author: lcawl

docs/en/install-upgrade/index.asciidoc

@@ -1,4 +1,3 @@
-[[elastic-stack]]
 = Elastic Stack
 
 :major-version: 5.x
@@ -11,6 +10,7 @@
 :logstashref:   https://www.elastic.co/guide/en/logstash/{branch}
 :xpackref:      https://www.elastic.co/guide/en/x-pack/{branch}
 
+[[elastic-stack]]
 == Overview
 
 The Elastic Stack is made up of multiple components developed by Elastic. In no particular