Docs: Cross-cluster search doc wasn't being included

docs/reference/modules.asciidoc

@@ -110,3 +110,6 @@ include::modules/threadpool.asciidoc[]
 include::modules/transport.asciidoc[]
 
 include::modules/tribe.asciidoc[]
+
+include::modules/cross-cluster-search.asciidoc[]
+