There is no way we can change this.
The subject of the email is copied from the name of the Google Form linked to your spreadsheet. Form Workflow works on the information stored in Google's memory. Google keeps in memory the information created at the very beginning of the form. Changing any information on the form at a later date will not change what is stored in Google's memory. Consequently, Form Workflow cannot reflect any changes you make to your form after its creation.
For example, even if you change the form name from Day-off Requests to Vacation Requests, the subject of the request-for-validation email will still show as "Form Workflow -Day-off Requests - Request for validation - FWXXXXXX" and NOT as "Form Workflow -Vacation Requests - Request for validation - FWXXXXXX."
To change the subject of the email, you need to to delete your current form with the wrong name.
Create a form from your spreadsheet first (Click on Tools in the menu, then Create a form). Put in this form all the information that you require.
Then start the Form Workflow add-on.