Fix incorrect variable name in `SMF.php` callback `callback_html()` 
* This caused the regular expression for BBCode `[list]` and `[list type=decimal]` to not be included in the `callback_html()` callback.

Props netweb. Fixes #2715

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5557 9866e705-20ec-0310-96e7-cbb4277adcfb