[DOS] Fix typo in CSV processor docs (#52649)

Corrects an example array in a snippet of the CSV processor docs.

docs/reference/ingest/processors/csv.asciidoc

@@ -23,7 +23,7 @@ include::common-options.asciidoc[]
 {
   "csv": {
     "field": "my_field",
-    "target_fields": ["field1, field2"]
+    "target_fields": ["field1", "field2"]
   }
 }
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