Hi,
the registration email is sent by Joomla, it isn't really Hikashop related, and if I'm not mistaken you're making use of a Fabrik plugin judging from your specific registration text.
All the email texts are defined as constants in language files, so you will need to locate the correct language file. The constant you're looking for is probably PLG_FABRIK_FORM_JUSER_AUTO_LOGIN_BODY, so you can search your files for it.
Once you've found it, it's better not to modify directly in the language file as your change will be undone again with a next update. Best make it a language override by going to your backend "Extensions -> Language(s) -> Overrides" and add your change there. Hope this helps.
Kind regards,
GW