Minor documentation bug around package.nw

[AshleyScirra]

As of NW.js 0.22.0 on macOS, using a file named package.nw in the Resources folder does not work. It does work if the file is called app.nw instead.

The documentation at http://docs.nwjs.io/en/latest/For%20Users/Package%20and%20Distribute/#package-your-app makes it sound like you have to use package.nw on macOS:

You can package all the files into a zip file and rename it as package.nw. And put it along with NW.js binaries for Windows and Linux. For Mac, put package.nw in nwjs.app/Contents/Resources/.

This should probably identify Mac as needing a different filename.