[DOCS] Remove unneeded comma from CSV processor example (#51859)

docs/reference/ingest/processors/csv.asciidoc

@@ -21,7 +21,7 @@ include::common-options.asciidoc[]
 {
   "csv": {
     "field": "my_field",
-    "target_fields": ["field1, field2"],
+    "target_fields": ["field1, field2"]
   }
 }
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