In order to override the text of the password reset emails in Keycloak, you want to do the following.

Make your own copy of the default themes. These can currently be found at: https://github.com/Halazv2/keycloak-theme/tree/master/themes/base/email 

Then, make changes to the theme as appropriate. There are several themes: the login theme stores the styling of the login pages. But the email theme contains the wording of the emails. 

Once you are satisfied with that changes that you've made, upload the theme and enable it for your realm. That's it!