Fix video URL to use master branch, not main

Author: m3nu

README.md

@@ -13,7 +13,7 @@
 
 Vorta is a backup client for macOS and Linux desktops. It integrates the mighty [BorgBackup](https://borgbackup.readthedocs.io) with your desktop environment to protect your data from disk failure, ransomware and theft.
 
-https://github.com/borgbase/vorta/raw/main/.github/vorta-screencast.mp4
+https://github.com/borgbase/vorta/raw/master/.github/vorta-screencast.mp4
 
 ## Why is this great? 🤩
 - **Encrypted, deduplicated and compressed backups** using [Borg](https://borgbackup.readthedocs.io) as backend.