fix getThumbnail promise

Signed-off-by: Npanqiangbiao <panqiangbiao@huawei.com>

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

@@ -1156,8 +1156,12 @@ async function example() {
     };
     const fetchFileResult = await media.getFileAssets(getImageOp);
     const asset = await fetchFileResult.getFirstObject();
-    asset.getThumbnail(size, (err, pixelmap) => {
+    asset.getThumbnail(size)
+    .then((pixelmap) => {
         console.info('mediaLibraryTest : getThumbnail Successfull '+ pixelmap);
+    })
+    .catch((err) => {
+        console.info('mediaLibraryTest : getThumbnail fail'+ err);
     });
 }
 ```