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 #25743, relates e3a0c11239

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 **/} },
     // ...
   ]
 }