[DOCS] Fixes directory attributes

Author: lcawl

docs/en/stack/index.asciidoc

@@ -13,6 +13,7 @@
 
 :kib-repo-dir:      {docdir}/../../../../kibana/docs
 :xes-repo-dir:      {docdir}/../../../../elasticsearch/x-pack/docs/en
+:es-repo-dir:      {docdir}/../../../../elasticsearch/docs
 
 include::{asciidoc-dir}/../../shared/versions63.asciidoc[]
 include::{asciidoc-dir}/../../shared/attributes.asciidoc[]