Typo & Pubctuation fix

[WweiL]

Minor fix

Is this a quick PR -- Typo, Quick Wording Fix, etc? If so, you don't need to follow the template -- please delete all and make the title of the PR representative of the changes and add a member of staff to review the PR. Thank you so much!

Description

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Related Issues

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Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• [ ] I updated AUTHORS with my name and email.
• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] My PR builds for all changed/updated/fixed behaviors (See [Contributor Guide]).
• [ ] I updated/added relevant documentation.
• [ ] The title of the PR is descriptive and doesn't have [WIP]
• [ ] I updated CHANGELOG.md to add a description of the change.
• [ ] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does this cause any new material not previously covered in the course to be required?

• [ ] Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md).
• [ ] No, this is not a breaking change.

[ezhang887]

Oh, I didn't realize this was merging into the st-arry-patch-2 branch instead of master branch. Can you make a PR again except into master?