Fix wrong icon position in form header

We made a mistake when adding the `calc()` function in commit 6df813fdb,
since the `/` operator originally only affected the `$heading-icon-size`
part of the operation, but affected the whole operation after that
commit. This caused the icon to be positioned on top of another icon.

app/assets/stylesheets/mixins/forms.scss

@@ -92,7 +92,7 @@
         font-size: rem-calc(44);
 
         &::after {
-          right: calc(#{$icon-size-with-padding + $polygon-size + $heading-icon-size} / 2);
+          right: calc(#{$icon-size-with-padding} + #{$polygon-size} + #{$heading-icon-size} / 2);
         }
 
         &::before {