Removing "Powered by WordPress" in a TwentyTen child theme

I want to remove the “proudly powered by WordPress” line that sits in the footer of the Twenty Ten theme. I know I can modify the theme’s footer.php file to do that. But since I have a child theme (which as yet changes nothing), I figured there must be a better way to do that in the child theme’s style.css file so that my changes are not lost when Twenty Ten is updated.

However, I have no idea at all how to do that. Can someone tell me how to do that using the child theme’s capabilities?

Thanks!