Update types_of_testing.md

[DivineDominion]

I corrected a few typos and re-formulated a few sentences to better match how I understand them. This could be wrong, of course :)

I also expanded the topic overviews a bit to frame the following explanations.

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@orta I'd love to include a few words on the difference between Unit Testing and Functional Testing/Integration Tests, but I'm not sure if understanding the difference is important for a pragmatic book.

[orta]

WRT:

I'd love to include a few words on the difference between Unit Testing and Functional Testing/Integration Tests, but I'm not sure if understanding the difference is important for a pragmatic book.

Pragmatically, it's XCTest vs UITesting

I covered this a bit in Integration Testing - but am definitely open to adding more around the mental model differences 👍

[orta]

And now that I've read the diff - I think this is a great improvement! Thanks @DivineDominion