Update ch4.md

[iwestlin]

val = yield *foo( val - 1 ); should be return yield *foo( val - 1 );

Yes, I promise I've read the Contributions Guidelines (please feel free to remove this line).

Specifically quoting these guidelines regarding typos:

Typos?

Please don't worry about minor text typos. These will almost certainly be caught during the editing process.

If you're going to submit a PR for typo fixes, please be measured in doing so by collecting several small changes into a single PR (in separate commits). Or, just don't even worry about them for now, because we'll get to them later. I promise.

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[iwestlin]

I am not accepting any contributions for 1st edition books.

I'm aware of this, but what about errata? Should I create a issue instead?

[getify]

The proposed change is incorrect.

[iwestlin]

@getify Are you sure about that? Have you run the code? Because I did...

Please kindly correct me if I was wrong or missing something...😅

[iwestlin]

Oh I think I get it now, the code actually want the result of request(...) and pass it to another request's ?v=${val}

I thought it just want to reduce 3 to 1 and then do the request...

Sorry for the misunderstanding, I'll close the PR right now...😢