Fix 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 an example
in the docs that is missing this.

Relates #25742

docs/reference/ingest/ingest-node.asciidoc

@@ -220,8 +220,8 @@ Here is the structure of a simulate request against an existing pipeline:
 POST _ingest/pipeline/my-pipeline-id/_simulate
 {
   "docs" : [
-    { /** first document **/ },
+    { "_source": {/** first document **/} },
-    { /** second document **/ },
+    { "_source": {/** second document **/} },
     // ...
   ]
 }