Update exec async doc - #185

The exec docs specify:

exec(command [, options] [, callback])
Available options (all false by default):
async: Asynchronous execution. Defaults to true if a callback is provided.
silent: Do not echo program output to console.

That seems to imply to me that the following code can be written:

exec("sleep 0.5", {async: false}, function() {
    echo("print first");
});
echo("print last");

which prints:

print last
print first

After looking at the code, however, it is clear that options.async is overridden to true if a callback is provided. This PR updates the README to be more in line with the code. What do you think?