fix(css): Fix missing % in calculation (#4157)

This bug has always been there, as far as I can tell, since the ads UI
was added.  This issue was caught by an internal compilation tool and
another project that consumes our UI.

ui/less/ad_controls.less

@@ -78,11 +78,11 @@
 
   /* This math is determining how far the button is from the right edge.
    * Ad panel's parent is centered and @bottom-controls-width wide, so
-   * 100 - @bottom-controls-width = margins from both sides of the container.
+   * 100% - @bottom-controls-width = margins from both sides of the container.
    * That divided by 2 is margin on one side, so we take that, and move the
    * button from its normal position to the right by that percentage.
    */
-  right: calc((100 - @bottom-controls-width) / 2 * -1);
+  right: calc((100% - @bottom-controls-width) / 2 * -1);
   display: flex;
   flex-direction: row;
   margin: 0;