Update _index.md (#5159)

Adding default value for line_format so customers know which is picked up if the option is not provided.

docs/sources/clients/fluentd/_index.md

@@ -255,7 +255,7 @@ There are few configurations settings to control the output format.
 
 - extra_labels: (default: nil) set of labels to include with every Loki stream. eg `{"env":"dev", "datacenter": "dc1"}`
 - remove_keys: (default: nil) comma separated list of needless record keys to remove. All other keys will be placed into the log line. You can use [record_accessor syntax](https://docs.fluentd.org/plugin-helper-overview/api-plugin-helper-record_accessor#syntax).
-- line_format: format to use when flattening the record to a log line. Valid values are "json" or "key_value". If set to "json" the log line sent to Loki will be the fluentd record (excluding any keys extracted out as labels) dumped as json. If set to "key_value", the log line will be each item in the record concatenated together (separated by a single space) in the format `<key>=<value>`.
+- line_format (default:key_value): format to use when flattening the record to a log line. Valid values are "json" or "key_value". If set to "json" the log line sent to Loki will be the fluentd record (excluding any keys extracted out as labels) dumped as json. If set to "key_value", the log line will be each item in the record concatenated together (separated by a single space) in the format `<key>=<value>`.
 - drop_single_key: if set to true and a record only has 1 key after extracting `<label></label>` blocks, set the log line to the value and discard the key.
 
 ### Buffer options