doc: improve child_process.execFile() code example

The current code example for child_process.execFile() doesn't work even on Unix-like platforms. Since there's no shell wrapper,

1. file must be a relative or absolute path to executable.
2. args must contain all the command line arguments for file.
3. I/O redirection and file globbing is not possible.

Here's an alternative I whipped up that should work on both Unix and Windows platforms. Suggestions welcome.