clean up code

README.md

@@ -80,7 +80,7 @@ if (!imagePage.isLast) {
 * Getting a Medium
 ```dart
 final Medium medium = await PhotoGallery.getMedium(
-  mediumId: "10", 
+  mediumId: "10",
   mediumType: MediumType.image
 );
 ```

lib/photo_gallery.dart

@@ -29,7 +29,7 @@ class PhotoGallery {
   }) async {
     final json = await _channel.invokeMethod('listAlbums', {
       'mediumType': mediumTypeToJson(mediumType),
-      'hideIfEmpty': hideIfEmpty
+      'hideIfEmpty': hideIfEmpty,
     });
     return json.map<Album>((x) => Album.fromJson(x)).toList();
   }