Update a URL to the video of general introduction to background processing on iOS system. (#577)

Co-authored-by: Kamil Komnacki <kamil.komnacki@j-labs.pl>

Author: kamilkomnacki

README.md

@@ -6,7 +6,7 @@
 
 Flutter WorkManager is a wrapper around [Android's WorkManager](https://developer.android.com/topic/libraries/architecture/workmanager), [iOS' performFetchWithCompletionHandler](https://developer.apple.com/documentation/uikit/uiapplicationdelegate/1623125-application) and [iOS BGAppRefreshTask](https://developer.apple.com/documentation/backgroundtasks/bgapprefreshtask), effectively enabling headless execution of Dart code in the background.
 
-For iOS users, please watch this video  on a general introduction to background processing: https://developer.apple.com/videos/play/wwdc2020/10063. All of the constraints discussed in the video also apply to this plugin.
+For iOS users, please watch this video  on a general introduction to background processing: https://developer.apple.com/videos/play/wwdc2019/707. All of the constraints discussed in the video also apply to this plugin.
 
 This is especially useful to run periodic tasks, such as fetching remote data on a regular basis.