Update `--reload_multifile_inactive_secs` default to 86400 in readme

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While the --reload_multifile_inactive_secs is already updated to 86400 (24 hours), multiple references in the root README.md still says 4000 seconds.
https://github.com/tensorflow/tensorboard/blob/12a30c324aed62dd4a5af13b4c5e27cbc03b10b0/tensorboard/plugins/core/core_plugin.py#L611

On a side note, the functionality was developed two years ago. Is it still 'experimental'?

Open README.md at the section on multiple or distributed summary writers and compare its --reload_multifile_inactive_secs references with the default shown in tensorboard/plugins/core/core_plugin.py. Update the outdated 4000-second references to 86400 seconds, then verify that all references in that README section are consistent; the experimental-status question is separate and needs a maintainer decision.