fix repository url

The repository.url property was incorrect, which causes this warning during installs `npm WARN package.json github-url-from-git No repository field`

This will also repository fix the link on the npm package page: https://www.npmjs.org/package/github-url-from-git
Merge pull request #14 from adamdbradley/patch-1

fix repository url. Awesome thanks, I'll release a patch to npm today.