Add missing backtick in comment

less/mixins.less

@@ -165,7 +165,7 @@
 
 // Backface visibility
 // Prevent browsers from flickering when using CSS 3D transforms.
-// Default value is `visible`, but can be changed to `hidden
+// Default value is `visible`, but can be changed to `hidden`
 // See git pull https://github.com/dannykeane/bootstrap.git backface-visibility for examples
 .backface-visibility(@visibility){
 	-webkit-backface-visibility: @visibility;