youtube plugin: Fix Maruku self-closing bug

Without this, the iframe element self closes, which is invalid and
causes the browser to think everything below it is part of the iframe
until it hits EOF.

From http://jekyllrb.com/docs/troubleshooting/#markup-problems:

> If you have an empty tag, i.e. <script src="js.js"></script>, Maruku
> transforms this into <script src="js.js" />. This causes problems in
> Firefox and possibly other browsers and is discouraged in XHTML. An
> easy fix is to put a space between the opening and closing tags.