Replace use of "working directory" with "working tree"

[bielsnohr]

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In episode 4 of section 1 ("Collaborative Software Development Using Git and GitHub"), the term "working directory" is used when describing the file space where the git project is checked out. The more accurate term for this is actually "working tree". There is a lot of confusion around this, including in git's own documentation. But based on this section, https://git-scm.com/book/en/v2/Getting-Started-What-is-Git%3F#The-Three-States, it seems like it is really "working tree" that should be used. Confusingly, the diagram is not up to date with the language in the documentation.

In any case, this is pretty minor, but I want to have it in an issue for eventual modification.