[improvement] Coupon: update cell value color (#2359)

packages/coupon-list/index.less

@@ -2,6 +2,10 @@
 @import '../style/mixins/ellipsis';
 
 .van-coupon {
+  &-cell--selected {
+    color: @text-color;
+  }
+
   &-list {
     height: 100%;
     position: relative;

packages/coupon-list/test/__snapshots__/index.spec.js.snap

@@ -18,7 +18,7 @@ exports[`coupon cell 2`] = `
   <div class="van-cell__title"><span>优惠券</span>
     <!---->
   </div>
-  <div class="van-cell__value"><span>-¥1.00</span></div> <i class="van-icon van-icon-arrow van-cell__right-icon">
+  <div class="van-cell__value van-coupon-cell--selected"><span>-¥1.00</span></div> <i class="van-icon van-icon-arrow van-cell__right-icon">
     <!---->
     <!----></i>
 </div>