chore: merge upstream (#35)

* chore: typos & refine comments (skyzh#65)

* typo in  week2-01-compaction.md

* chroe: typos & add comments

* chore: more typos

* Update week2-01-compaction.md

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Co-authored-by: Alex Chi Z <[email protected]>

* Fix typos in W3D5 writeup and code (skyzh#67)

* Fix minor mistake in W3D6 writeup (skyzh#69)

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Co-authored-by: Alex Chi Z <[email protected]>
Co-authored-by: Yue Yin <[email protected]>