Microdata in single article view causes invalid HTML

### Issue
With #3358 I implemented hardcoded Microdata into the layouts. However I used a `<span>` to assign the `articleBody` item property. Since spans aren't allowed to contain block elements, this causes now a validation error when checking with the W3C validator.
See Issue #3508 and Tracker http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33662

### Solution
Using `<section>` instead of the `<span>` like suggested by @betweenbrain in the original PR solves the issue.

### Testing
Validate the single article view with Joomla 3.3 RC and verify the error. Apply this PR and check again. It should verify and the template should not change in any way.