removed reference to button color and gradient

Changed the wording to reflect the default button's new look since it's no longer grey and doesn't have a gradient. I'd say this new wording isn't ideal and needs something a bit more, well, descriptive.

css.html

@@ -1972,7 +1972,7 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
       <button type="button" class="btn btn-link">Link</button>
     </div>
 {% highlight html %}
-<!-- Standard gray button with gradient -->
+<!-- Standard button  -->
 <button type="button" class="btn btn-default">Default</button>
 
 <!-- Provides extra visual weight and identifies the primary action in a set of buttons -->