Fixes the `require_once` path for `class-vip-tfpdf.php`

[anaemnesis]

Reported at https://wordpress.org/support/topic/bug-in-version-2-3-0/.

Changes proposed in this Pull Request

Adds a slash to the require_once that tries to load the path: class-vip-tfpdf.php

https://github.com/woocommerce/sensei-certificates/blob/bb17712bb25ce2766ca82320f7cf5b4e50ff4c82/classes/class-woothemes-sensei-certificates-tfpdf.php#L26

Without this slash, the path is wp-content/plugins/sense-certificates/classesclass-vip-tfpdf.php, per the error shown:

Testing instructions

• Check out branch
• View certificates to ensure error is resolved, or
• Load up the file in your IDE and have it double-check the loaded path