updating pull requests link

have contributors who want to learn more about contributing go [here](https://help.github.com/articles/using-pull-requests) instead of [here](https://help.github.com/articles/sending-pull-requests) in the `contributing.md`.

this is the [same error](https://github.com/h5bp/html5-boilerplate/pull/1308) that was fixed in the html5 boilerplate