about dialog: use white scrollbar instead of the default since we using a dark background

Sources/VLCAboutViewController.m

@@ -31,6 +31,7 @@
     _webView.clipsToBounds = YES;
     _webView.delegate = self;
     _webView.backgroundColor = [UIColor VLCDarkBackgroundColor];
+    _webView.scrollView.indicatorStyle = UIScrollViewIndicatorStyleWhite;
     self.view = _webView;
 }