Tiny Yslow description fix (#527)

The Firebug add-on doesn't exist anymore, nor does the extension Yslow for it, so it's time for a tiny wording change in the Yslow description.

wp-cache.php

@@ -1273,7 +1273,7 @@ function toggleLayer( whichLayer ) {
 			<p><?php _e( 'Caching is only one part of making a website faster. Here are some other plugins that will help:', 'wp-super-cache' ); ?></p>
 			<ul style="list-style: square; margin-left: 2em;">
 			<li><?php printf( __( '<a href="%s">Jetpack</a> provides everything you need to build a successful WordPress website including an image/photo CDN (free) and a video hosting service (paid).', 'wp-super-cache' ), 'https://jetpack.com/redirect/?source=jitm-wpsc-recommended' ); ?></li>
-			<li><?php printf( __( '<a href="%s">Yahoo! Yslow</a> is an extension for the Firefox add-on Firebug. It analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages. Also try the performance tools online at <a href="%s">GTMetrix</a>.', 'wp-super-cache' ), 'http://yslow.org/', 'https://gtmetrix.com/' ); ?></li>
+			<li><?php printf( __( '<a href="%s">Yahoo! Yslow</a> analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages. Also try the performance tools online at <a href="%s">GTMetrix</a>.', 'wp-super-cache' ), 'http://yslow.org/', 'https://gtmetrix.com/' ); ?></li>
 			<li><?php printf( __( '<a href="%s">Use Google Libraries</a> allows you to load some commonly used Javascript libraries from Google webservers. Ironically, it may reduce your Yslow score.', 'wp-super-cache' ), 'https://wordpress.org/plugins/use-google-libraries/' ); ?></li>
 			<li><?php printf( __( '<strong>Advanced users only:</strong> Install an object cache. Choose from <a href="%s">Memcached</a>, <a href="%s">XCache</a>, <a href="%s">eAcccelerator</a> and others.', 'wp-super-cache' ), 'https://wordpress.org/plugins/memcached/', 'https://neosmart.net/WP/XCache/', 'https://neosmart.net/WP/eAccelerator/' ); ?></li>
 			<li><?php printf( __( '<a href="%s">Cron View</a> is a useful plugin to use when trying to debug garbage collection and preload problems.', 'wp-super-cache' ), 'https://wordpress.org/plugins/cron-view/' ); ?></li>