simple string replace? (first plugin)

So i’m trying to fix some minor problems with a clean 3.3 install and the TwentyEleven template (1.3)

the first error i want to fix is the

<a href="http://wordpress.org/" title="Semantic Personal Publishing Platform" rel="generator">Proudly powered by WordPress</a>

and change rel=”generator” to rel=”author” with the code below but it’s not working (note this is my first attempt at a plugin)

<?php
function replace_content_rel_generator($content) {
$content = str_replace("rel='generator'", "rel='author'", $content);
return $content; }
add_filter('wp_footer','replace_content_rel_generator');
?>