update the std::io::pipe() example to comment deadlocks

[oconnor663]

The previous version of the example wasn't a deadlock, but only because the Command object was a temporary. That's quite subtle, and I think it's better to call out the potential deadlock explicitly.

Rendered

[senekor]

I think this is too big of a change. The post has been released and there isn't anything necessarily wrong with the example. The blog is not documentation that evolves and is polished over time. Post-release updates are usually about fixing typos, broken URLs and so on.