Chapter 4.1 - consider change to punctuation to make sentence clearer

[hughrun]

There is potentially confusing punctuation in this section of Chapter 4.1:

Stack-Only Data: Copy

There’s another wrinkle we haven’t talked about yet. This code using integers, part of which was 
shown in Listing 4-2, works and is valid:

On my initial read this looked like a typo which should have read This code is using integers...

I suggest replacing the commas with dashes to make the sentence clearer:

This code using integers – part of which was shown in Listing 4-2 – works and is valid:

[steveklabnik]

Sure, I would happily take a PR that uses an em dash here.