Update video_player readme to change sample video to https (flutter#3546)

Author: jeremiahlukus

packages/video_player/video_player/README.md

@@ -77,7 +77,7 @@ class _VideoAppState extends State<VideoApp> {
   void initState() {
     super.initState();
     _controller = VideoPlayerController.network(
-        'http://www.sample-videos.com/video123/mp4/720/big_buck_bunny_720p_20mb.mp4')
+        'https://www.sample-videos.com/video123/mp4/720/big_buck_bunny_720p_20mb.mp4')
       ..initialize().then((_) {
         // Ensure the first frame is shown after the video is initialized, even before the play button has been pressed.
         setState(() {});