Fixing small typo in Calculus Chapter 6

[Bongo50]

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:

-fixing a small typo of "the the" to "to the"

Sorry for submitting 2 pull requests to the same file in quick succession. I noticed these issues only as I was reading the lesson and submitted these pull requests as soon as I saw them.

[kurtbruns]

Thanks for pointing out and fixing the typo!