Merge pull request #599 from ably/fix-readme-image

docs: update image reference in README and rename file

Author: ttypic

README.md

@@ -1,4 +1,4 @@
-![Ably Pub/Sub Flutter Header](images/flutterSDK-github.png)
+![Ably Pub/Sub Flutter Header](images/flutter-github.png)
 [![Pub Version](https://img.shields.io/pub/v/ably_flutter)](https://pub.dev/packages/ably_flutter)
 [![License](https://badgen.net/github/license/ably/ably-flutter)](https://github.com/ably/ably-flutter/blob/main/LICENSE)