Add wait for yellow to doc snippet so it runs cleanly

Found by http://build-us-00.elastic.co/job/es_core_master_window-2008/3866/console

docs/reference/query-dsl/nested-query.asciidoc

@@ -22,6 +22,8 @@ PUT /my_index
         }
     }
 }
+
+GET _cluster/health?wait_for_status=yellow
 --------------------------------------------------
 // CONSOLE
 // TESTSETUP