Update HTML tag to fit screen sizes starting at 320px

When viewing the 24pullrequest.com/contributing page on a 320px-width screen, the "Contributing" tag continues off the regular flow of the page. 

![320px - ios](https://f.cloud.github.com/assets/2647488/1650037/bfe9dc4a-5a54-11e3-80e6-73a903d70832.png)

By changing its HTML tag from h1 to h2, it fits without exceeding the width of the page.