Makes more obvious that we expect an array

docs/plugins/ingest-attachment.asciidoc

@@ -52,7 +52,7 @@ The node must be stopped before removing the plugin.
 | `field`                | yes       | -                | The field to get the base64 encoded field from
 | `target_field`         | no        | attachment       | The field that will hold the attachment information
 | `indexed_chars`        | no        | 100000           | The number of chars being used for extraction to prevent huge fields. Use `-1` for no limit.
-| `properties`           | no        | all properties   | Properties to select to be stored. Can be `content`, `title`, `name`, `author`, `keywords`, `date`, `content_type`, `content_length`, `language`
+| `properties`           | no        | all properties   | Array of properties to select to be stored. Can be `content`, `title`, `name`, `author`, `keywords`, `date`, `content_type`, `content_length`, `language`
 | `ignore_missing`       | no        | `false`          | If `true` and `field` does not exist, the processor quietly exits without modifying the document
 |======