Update _mixins.scss

Changed "overdue" to "overdo." Simple grammatical edit in comments.

library/scss/_mixins.scss

@@ -170,7 +170,7 @@ USAGE: @include transition(all 0.2s ease-in-out);
 /*********************
 CSS3 GRADIENTS
 Be careful with these since they can
-really slow down your CSS. Don't overdue it.
+really slow down your CSS. Don't overdo it.
 *********************/
 
 /* @include css-gradient(#dfdfdf,#f8f8f8); */