Correct license field in package.json

A tiny update to the license field!
See [spec](https://www.npmjs.org/doc/files/package.json.html#license), [validator](https://github.com/gorillamania/package.json-validator/blob/08c9223a4c402b732b172fbf7b6dd11ef80ea6e7/PJV.js#L27).