Advanced Selectors: Add CSS Speedrun to assignment list

[cyphersept]

Because

CSS Diner is mainly oriented towards teaching how to use specific advanced selectors to solve a problem where the student is already told what selector to use. CSS Speedrun provides more practice identifying which advanced selector to use for a given situation. It's 10 problems long and can be completed within 10 minutes, making it a good review.

This PR

• Adds CSS Speedrun to the end of the assignments list

Issue

None

Additional Information

None

Pull Request Requirements

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• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
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