Update README.md to point to flutter.dev (#8557)

Update from flutter.io to flutter.dev domain.

README.md

@@ -7,7 +7,7 @@ Flutter is optimized for today's, and tomorrow's, mobile devices. We are
 focused on low-latency input and high frame rates on Android and iOS.
 
 The Flutter Engine is a portable runtime for hosting
-[Flutter](https://flutter.io) applications.  It implements Flutter's core
+[Flutter](https://flutter.dev) applications.  It implements Flutter's core
 libraries, including animation and graphics, file and network I/O,
 accessibility support, plugin architecture, and a Dart runtime and compile
 toolchain. Most developers will interact with Flutter via the [Flutter
@@ -16,9 +16,9 @@ reactive framework, and a rich set of platform, layout and foundation widgets.
 
 If you are new to Flutter, then you will find more general information
 on the Flutter project, including tutorials and samples, on our Web
-site at [flutter.io](https://flutter.io). For specific information
+site at [Flutter.dev](https://flutter.dev). For specific information
 about Flutter's APIs, consider our API reference which can be found at
-the [docs.flutter.io](https://docs.flutter.io/).
+the [docs.flutter.dev](https://docs.flutter.dev/).
 
 If you intend to contribute to Flutter, welcome! You are encouraged to
 start with [our contributor