Description of the Change

In this PR I'm fixing a PHP fatal error after a missed constant renaming.

Closes #246

Verification Process

1. Activate the plugin for the first time on site or update the simple_podcasting_onboarding option to no, if it is already in use on site.
2. Feel a podcast show details on onboarding and click on create.
3. Verify that when the onboarding finishes there is no PHP warning in the debug log and that there is no notice about the failed creation of the podcast because of an "Invalid taxonomy" error.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed - Fixed a PHP warning when creating a new podcast.

Credits

Props @kmgalanakis