[nrktv:series] Fix extraction

[mpolden]

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• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

NRK has made a minor change to their site and downloading series no longer works. This change fixes it and TestDownload.test_NRKTVSeason now passes.