modify the sample code of readPixels

Signed-off-by: Nxu-rui-w <xurui101@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-image.md

@@ -169,7 +169,12 @@ readPixels(area: PositionArea): Promise\<void>
 **示例：**
 
 ```js
-const area = new ArrayBuffer(400);
+const area = {
+    pixels: new ArrayBuffer(8),
+    offset: 0,
+    stride: 8,
+    region: { size: { height: 1, width: 2 }, x: 0, y: 0 }
+}
 pixelmap.readPixels(area).then(() => {
     console.log('Succeeded in reading the image data in the area.'); //符合条件则进入
 }).catch(error => {