How Do I Change 404 Error Page Title (The One in the Tab)

Hi.

I want to change the “Nothing found for” text in the tab that shows when a 404 error is encountered. I’ve searched around the in need of a solution, and I saw one that said to put `if( is_404() ) echo ‘404 message goes here | ‘;
else wp_title( ‘|’, true, ‘right’ );in the place ofwp_title( ‘|’, true, ‘right’ )` in header.php. I tried this, but it had no effect. It still says “Nothing found for <search term>”. I’ve tried making small edits to the code like putting in spaces, indents, apostrophes, etc. but nothing worked. How can I achieve this? I’m using the Twenty Eleven theme and my site is here.