Update example in README
Update example in README

[`parseFilename`](https://github.com/andrewrk/node-multiparty/blob/master/index.js#L765) don't return a `null` (but `undefined` value) when part is a field. Updated the README to get a working example.
andrewrk/node-multiparty,pillarjs/multiparty,yan5845hao/node-multiparty