scss/_carousel.scss: Add missing backticks in comment

[ci skip]

scss/_carousel.scss

@@ -179,7 +179,7 @@
     // IE9 hack for event handling
     //
     // Internet Explorer 9 does not properly handle clicks on elements with a `background-color` of `transparent`,
-    // so we use rgba(0,0,0,0) instead since it's a non-buggy equivalent.
+    // so we use `rgba(0,0,0,0)` instead since it's a non-buggy equivalent.
     // See https://developer.mozilla.org/en-US/docs/Web/Events/click#Internet_Explorer
     background-color: rgba(0,0,0,0); // IE9
     border: 1px solid $carousel-indicator-border-color;