update the theme() related documentation to drupal 7 theme function signature

There is an outdated example of how to use the theme function to print an swf object. It uses the D6 theme function signature.

There is even a bug report related to this misleading example: http://drupal.org/node/1259682

Here is a tiny patch, that changes one line. It is my very first one for Drupal, maybe that explains why such a tiny patch even bothers me. It seems to be a low hanging fruit :) But let's see, I am happy on any feedback :)

Best,
walter