add missing @Override

weixin-java-mp/src/main/java/me/chanjar/weixin/mp/bean/custombuilder/WxCardBuilder.java

@@ -23,6 +23,7 @@ public final class WxCardBuilder extends BaseBuilder<WxCardBuilder> {
     return this;
   }
 
+  @Override
   public WxMpCustomMessage build() {
     WxMpCustomMessage m = super.build();
     m.setCardId(this.cardId);