[youtube] Fix XSRF Token regex

[DonneyF]

---

Before submitting a pull request make sure you have:

• [x] At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• [x] Searched the bugtracker for similar pull requests
• [x] Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• [ ] I am the original author of this code and I am willing to release it under Unlicense
• [x] I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• [x] Bug fix
• [ ] Improvement
• [ ] New extractor
• [ ] New feature

---

Description of your pull request and other information

This PR fixes failing XSRF Token extraction from the YouTube extractor due to outdated regex. Regex for fix is taken from https://github.com/ytdl-org/youtube-dl/commit/942b8ca3bee90e13e0d058c03248670be1a9991c

[pukkandan]

#280 already has a patch for this