Clarifying data download in "Things You'll Need"

[chris-prener]

@sheraaron suggests the following change to the intro page:

I also suggest to add to https://datacarpentry.org/spreadsheets-socialsci/00-intro/index.html Under Things you'll need to complete this tutorial: Open a folder on your computer and save all your spreadsheets necessary for this exercise.

This makes sense to me - any objections to this change @ErinBecker and/or @fmichonneau?

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This was originally submitted as issue #71

[ErinBecker]

Sounds good to me! Maybe change "exercise" to "lesson"?

[chris-prener]

Great - will put this in the next milestone