Hi Davide,
Thanks for reaching out!
That is a file which cannot be overridden, it is not set up for template overrides.
Apart from modifying the core files you can set up a “translation” for that string.
Depending on the language of your website open the corresponding event-tickets-[language]_[LANGUAGE].po file, look for the string, add the translation, save and done.
For UK English the file would be event-tickets-en_GB.po. If you need one for American English, then you will need to create a file / translation for that: event-tickets-en_US.po
For editing translations you can use the Loco Translate plugin directly on your site or you can use the PoEdit software (free). In this latter case you will need to upload the translations to your server.
Here is some more information on translations that might be useful:
Hope this helps. Let me know if you have further questions.
Cheers,
Andras