upd(uploader): docs & tips style

src/packages/uploader/doc.md

@@ -21,6 +21,8 @@ app.use(Uploader);
 
 ``` html
 <nut-uploader url="http://服务器地址"></nut-uploader>
+```
+
 ### 直接调起摄像头
     
 ``` html

src/packages/uploader/index.scss

@@ -48,6 +48,7 @@
         bottom: 0;
         left: 0;
         right: 0;
+        text-align: center;
         font-size: 12px;
         color: $white;
         height: 30px;