_sass: Fix calculation error in abstract-card margin

_sass/custom/card.scss

 @mixin abstract-card() {
   box-shadow: 0 1px 3px rgba(0, 0, 0, 0.07), 0 4px 14px rgba(0, 0, 0, 0.05);
-  margin: $sp-4 -$gutter-spacing-sm;
+  margin: $sp-4 (-$gutter-spacing-sm);
 
   @include mq(md) {
     border-radius: $border-radius;