Fix "Do You Know CSS Fundamentals?" question 10

[MartinRosenberg]

Question 10 asks how to center a block vertically, and says that margin: auto is the answer—but auto margins can only center a block horizontally, not vertically. Thankfully, as of earlier in 2024, align-content can now apply to flow layout as well as flexbox and grid.

[netlify[bot]]

👷 Deploy request for danlevy accepted.

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[justsml]

Thanks for the fix @MartinRosenberg ! I kept going back and forth on how many old vs. new questions to add and think I mangled the content in there.