fix tag

Author: JPryce-Aklundh

modules/ROOT/pages/subqueries/call-subquery.adoc

@@ -579,7 +579,7 @@ Call subqueries can be used to further process the results of a xref:clauses/uni
 This example query finds the youngest and the oldest `Player` in the graph.
 
 .Find the oldest and youngest players
-// tag::subqueries_call_subquery_optional_union[]
+// tag::subqueries_call_subquery_union[]
 [source, cypher]
 ----
 CALL () {
@@ -595,7 +595,7 @@ UNION
 }
 RETURN p.name AS playerName, p.age AS age
 ----
-// end::subqueries_call_subquery_optional_union[]
+// end::subqueries_call_subquery_union[]
 
 .Result
 [role="queryresult",options="header,footer",cols="2*<m"]