Fix typo
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I found some typo, so I fixed them.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because it's just a fix of typo.
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20096)
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554b0c0d06
