Fix for collapsible sections width issue

With the fix introduced in [#68](https://github.com/alphagov/manuals-frontend/pull/68), there was an issue were it was finding the first element which wasn't a h2, when it should have been starting at the first element and doing nothing if it was a h2. This commit fixes that bug.
`!firstChild.is('h2')` would be more jQuery-esque