Fix Tutorial playground for removal of `ArrayOf<>`

[danj-stripe]

What's in this pull request?

This case was changed from ArrayOf<Int> to [Int], but the corresponding getArray stuck around.

From #264, on this line https://github.com/CodaFi/SwiftCheck/commit/b6b4bc61dc0a41889040a38ebe8f5926ae713d2c#diff-47970fd31917fb63ccf81bd68065f0abR343

Thanks for the hard work on this!

Why merge this pull request?

Tutorial fails to run without it:

error: Tutorial.playground:339:9: error: value of type '[Int]' has no member 'getArray'
        return xs.getArray.filter(f).reduce(true, { $0 && f($1) })
               ^~ ~~~~~~~~

What's worth discussing about this pull request?

N/A

What downsides are there to merging this pull request?

N/A

[CodaFi]

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