doc: update fs callback example error stack

Update the error stack printed in the default callback
example in the fs doc, matching the latest lib/fs.js.
Truncates the stack to make it easier to update in the future.

PR-URL: https://github.com/nodejs/node/pull/6617


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>