chore: updated CHANGELOG.md

packages/stream_chat_flutter/CHANGELOG.md

@@ -4,7 +4,11 @@
 
 - [#516](https://github.com/GetStream/stream-chat-flutter/issues/516): Added `StreamChatThemeData.placeholderUserImage` for 
   building a widget when the `UserAvatar` image is loading
-- Added a `backgroundColor` property to the various header widgets
+- Added a `backgroundColor` property to the following widgets:
+  - `ChannelHeader`
+  - `ChannelListHeader`
+  - `GalleryHeader`
+  - `ThreadHeader`
 
 
 🔄 Changed