Caldera Forms Rich Editor (WYSIWYG) fields are powered by the Trumbowyg JavaScript library. If you would like to modify the options passed to Trumbowyg, you can use the caldera_forms_wysiwyg_options filter.
See: https://alex-d.github.io/Trumbowyg/documentation.html
Where Does This Code Go?
When using WordPress hooks to customize Caldera Forms or other plugins you should not modify the plugin files, or you will lose your changes when you update the plugin. Instead you should create a small plugin to hold the custom code. It's easy, learn how here.
Technically you can add the custom code to your theme's functions.php, but then you will not be able to change your theme and keep these customizations.