Update package.json

Fixed typo.

According to http://package-json-validator.com/ the package.json is invalid and is lacking an `author` field (I see that you have a plural `authors` field though).

Per https://npmjs.org/doc/json.html:

> The "author" is one person. "contributors" is an array of people. A "person" is an object with a "name" field and optionally "url" and "email", like this: