update settings for decode_csv_fields (#15249) (#15550)

Co-authored-by: DeDe Morton <dede.morton@elastic.co>

Co-authored-by: Sophia Xu <sophia.xu@elastic.co>

libbeat/processors/decode_csv_fields/docs/decode_csv_fields.asciidoc

@@ -13,7 +13,7 @@ processors:
  - decode_csv_fields:
      fields:
         message: decoded.csv
-     separator: ,
+     separator: ","
      ignore_missing: false
      overwrite_keys: true
      trim_leading_space: false