Update documentation around index.look_ahead_time setting. (#103975)

Adjusted the default after #103898

docs/reference/data-streams/tsds-index-settings.asciidoc

@@ -28,7 +28,7 @@ value (exclusive) accepted by the index. Only indices with an `index.mode` of
 `index.look_ahead_time`::
 (<<_static_index_settings,Static>>, <<time-units,time units>>)
 Interval used to calculate the `index.time_series.end_time` for a TSDS's write
-index. Defaults to `2h` (2 hours). Accepts `1m` (one minute) to `2h` (two
+index. Defaults to `30m` (30 minutes). Accepts `1m` (one minute) to `2h` (two
 hours). Only indices with an `index.mode` of `time_series` support this setting.
 For more information, refer to <<tsds-look-ahead-time>>. Additionally this setting
 can not be less than `time_series.poll_interval` cluster setting.