[doc] minor fix in the LESS mixins example

css.html

@@ -501,7 +501,7 @@ base_url: "../"
   }
 }
 
-// Generate the large column offsets
+// Generate the medium column offsets
 .make-md-column-offset(@columns) {
   @media (min-width: @screen-medium) {
     margin-left: percentage((@columns / @grid-columns));