Fix 2nd DuplexStream example to be like the first

The syntax changes without explanation in the 2nd example, and the
changes throw compile errors, so I made them be like the first example
that works.
Only send one value in the first ex to match the text

The explanation for this code example says:

> In the main function, we make a `DuplexStream`, send one end to a
> new task, and then send it a `22`, and print out the result.
> Because this task is running in the background, we can send it
> bunches of values:

But the way this example was written, it was already sending bunches of
values. So I changed the first example to only send 22 and then the
contrast between sending 1 and sending bunches is clearer.
Change try_send to send

I don't know why some of these were try_send and some were send, and
send seems to work, so I changed them all.

If it needs to be the way it is for some reason, I think there should be
an explanation of the reason.
End the child task before sending a value back

This makes more sense to me-- otherwise it's not dying when you send it
0, it's sending back 1 and *then* dying. If that was intended, I think
there should be an explanation about why it was intended.
Merge pull request #64 from carols10cents/duplexstream

Duplexstream