fix js crash of ng waterflow demo

Signed-off-by: Nzcdqs <junfeng.lijunfeng@huawei.com>
Change-Id: I1fbbfcff7b8dd4c2be401bbc18897bae58f4f4b8

zh-cn/application-dev/reference/arkui-ts/ts-container-waterflow.md

@@ -250,7 +250,7 @@ struct WaterflowDemo {
 
   build() {
     Column({ space: 2 }) {
-      WaterFlow({ footer: this.itemFoot, scroller: this.scroller }) {
+      WaterFlow({ footer: this.itemFoot.bind(this), scroller: this.scroller }) {
         LazyForEach(this.datasource, (item: number) => {
           FlowItem() {
             Column() {