[DOCS] Added missing attribute to specify the location of the included tests.

docs/java-rest/index.asciidoc

@@ -1,6 +1,8 @@
 [[java-rest]]
 = Java REST Client
 
+:doctest:   {docdir}/../../client/rest-high-level/src/test/java/org/elasticsearch/client/documentation
+
 include::../Versions.asciidoc[]
 
 include::overview.asciidoc[]