Fixing the order of the footnotes in Calculus Chapter 6

[Bongo50]

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This pull request:

-fixes the order of the footnotes in Calculus Chapter 6

Sorry if I've not done this correctly. I'm not very experienced with Github.

[kurtbruns]

Thanks for correcting the order of the footnotes! This is perfect, by requesting a pull request I can review what you've changed in the lesson before merging it into the main branch