Fix readme link

The main issue with your link is that, while github understands the format, the [npm](https://www.npmjs.org/package/humanize) readme display does not. It gives the broken link: http://en.wikipedia.org/wiki/Ordinal_number_/(linguistics/

Note that parenthesis are invalid in a url anyways and should always be percent encoded, as this pull request does.

Thanks for the great project!