doc: improve example docs/code/progress/main.c

[bnoordhuis]

From the example:

percentage = downloaded*100.0/size;
async.data = (void*) &percentage;
uv_async_send(&async);

Multiple problems:

1. uv_async_send() is not a memory barrier
2. percentage goes out of scope when the thread exits, which can happen before the async callback runs
3. An optimizing compiler may opt to do the calculation in place, meaning other threads may see a partial result

There's a sleep(1) right after so (2) is not super likely but it's still a bad example.

[juanarbol]

You want is an example of async use? Or a different one from this one