Improve consistency of subheading styling on Curation pages. Fixes #555

Just added ```class="instructions"``` (and in one case moved the relevant ```p```) to get the border styling between the H3 and paragraph tags.

![curation_-_about_pages___default_exhibit_-_blacklight](https://f.cloud.github.com/assets/101482/2432782/ef482e02-ad73-11e3-8ab2-16ec875254e0.png)