The point is that i've already installed that update. What do you think that means ?
Please notice the information at the bottom of the page https://www.dokuwiki.org/update_check
the page update_check wrote:
When upgrading your installation, it might happen that DokuWiki continues to show the update message, even though the number in doku.php was increased by the upgrade. This is because DokuWiki caches already fetched messages for a day and will only refetch if the last modified timestamp of doku.php is higher than the one of the cache file. To stop the outdated update message from showing you can simply wait a day, touch the doku.php or delete the data/cache/messages.txt cache file.
If for security reason you renamed the data repository, let say xyzdata for instance then the cache file is to be found at xyzdata/cache/messages.txt even if some install re-created “data” and “data/cache” repositories.