Fix another simulate example in ingest docs

When simulating an ingest pipeline against an existing pipeline, the _source field is required to wrap each doc. This commit fixes another example in the docs that is missing this. Relates elastic#25743, relates e3a0c11

Author: GlenRSmith

docs/reference/ingest/ingest-node.asciidoc

@@ -205,8 +205,8 @@ POST _ingest/pipeline/_simulate
  // pipeline definition here
  },
  "docs" : [
- { /** first document **/ },
- { /** second document **/ },
+ { "_source": {/** first document **/} },
+ { "_source": {/** second document **/} },
  // ...
  ]
 }