Make function name correct

[zachary-russell]

The function is described in this flow is the uniqueWords() wrapper function not wordsUsed() I changed the name to be more clear as it confused me on the first pass.

[getify]

I understand where your confusion came from, but this change is problematic because the whole section of this chapter flows from using wordsUsed as the name to then uniqueWords. I'm not recalling why I made that shift at this point, but it seems like it was intentional.

I'm going to hold off on this change until revisiting it in second edition. If the name change is important, it should be described in the text. If it's not, then the same name should be used throughout the whole chapter.

Thanks for bringing it up.