Small grammatical fix to index.mdx

[mcintyret]

Lesson authoring peer review checklist:

• [ ] Includes authorship line: Text adaptation by [Name]
• [ ] The lesson has an informative/compelling description, fit for a preview on the home page.
• [ ] The lesson has no grammar/spelling errors
• [ ] The lesson has no mathematical errors
• [ ] The table of contents effectively conveys the overall structure of the lesson
• [ ] The lesson uses no # Top Level headings
• [ ] The lesson includes well-chosen comprehension questions
• [ ] The lesson incorporates the core visuals from the video
• [ ] The visuals are clear and helpful for understanding
• [ ] The components are used in the places and ways they are intended (see the readme)

Website change review checklist:

• [ ] I have requested a review from the appropriate persons
• [ ] I have checked that my changes work in the preview
• [ ] I have checked that the rest of the site is still functioning in the preview
• [ ] I have checked that my content/code is consistent with existing content/code
• [ ] I have used components in the places and ways they are intended (see the readme)
• [ ] I have formatted all of my code with Prettier

This pull request:

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[3b1b]

Thank you!