Add a wait_for_stats=yellow to a docs snippet

It was making unstable tests.

docs/reference/query-dsl/parent-id-query.asciidoc

@@ -28,6 +28,8 @@ PUT /my_index
         }
     }
 }
+
+GET /_cluster/health?wait_for_status=yellow
 ------------------------------------------
 // CONSOLE
 // TESTSETUP
@@ -73,7 +75,7 @@ This query has two required parameters:
 
 `id`::    The required parent id select documents must referrer to.
 
-`ignore_unmapped`::  When set to `true` this will ignore an unmapped `type` and will not match any 
+`ignore_unmapped`::  When set to `true` this will ignore an unmapped `type` and will not match any
 documents for this query. This can be useful when querying multiple indexes
 which might have different mappings. When set to `false` (the default value)
 the query will throw an exception if the `type` is not mapped.