Correct the Typescript defs for turf.voronoi.

[stevage]

Fixes #1422.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Have read How To Contribute.
• Run npm test at the sub modules where changes have occurred.
• Run npm run lint to ensure code style at the turf module level.

[Naitik333]

Can any one please merge Pull request, its open from Jun 24, 2018. Its stopping us from using turfJs voronoi in typescript environment.

[mfedderly]

Good catch, this is a breaking change but it is also just wrong on master so lets get it merged to fix that.