fixes #14714: don't use specific value in buttons docs

docs/_includes/css/buttons.html

@@ -114,7 +114,7 @@
 
 
   <h2 id="buttons-disabled">Disabled state</h2>
-  <p>Make buttons look unclickable by fading them back 50%.</p>
+  <p>Make buttons look unclickable by fading them back with <code>opacity</code>.</p>
 
   <h3>Button element</h3>
   <p>Add the <code>disabled</code> attribute to <code>&lt;button&gt;</code> buttons.</p>