Fix UI bug on category selector. reviewed by redding.

@@ -56,7 +56,7 @@ export default class CategorySelector extends Component {
         }
 
         // 位于列表尾，且（容器宽度-x坐标）< 屏幕x中心距离
-        if ((this.scrollContentWidth - rowX - 10) < width / 2) {
+        if ((this.scrollContentWidth - rowX - rowWidth/2) < width / 2) {
             this.listView && this.listView.scrollTo({x: this.scrollContentWidth - width, y: 0, animated: true});
             return;
         }