Update index.mdx - Githubissues

Just changed the A21 element of the array.

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:

Changed one element of the the matrix as it wasn't representing the first column as per example

3b1b / 3Blue1Brown.com