Update links to platform services -> channels (#8975)

examples/platform_services/README.md

 # Example of calling platform services from Flutter
 
-This project demonstrates how to connect a Flutter app to platform specific services. 
+This project demonstrates how to connect a Flutter app to platform specific services.
 
 You can read more about
-[accessing platform and third-party services in Flutter](https://flutter.io/platform-services/).
+[accessing platform and third-party services in Flutter](https://flutter.io/platform-channels/).
 
 ## iOS
 You can use the commands `flutter build` and `flutter run` from the app's root

packages/flutter/lib/src/services/platform_channel.dart

@@ -26,7 +26,7 @@ import 'platform_messages.dart';
 /// one message handler can be registered with the channel name at any given
 /// time.
 ///
-/// See: <https://flutter.io/platform-services/>
+/// See: <https://flutter.io/platform-channels/>
 class PlatformMessageChannel<T> {
   /// Creates a [PlatformMessageChannel] with the specified [name] and [codec].
   ///
@@ -102,7 +102,7 @@ class PlatformMessageChannel<T> {
 /// The identity of the channel is given by its name, so other uses of that name
 /// with may interfere with this channel's communication.
 ///
-/// See: <https://flutter.io/platform-services/>
+/// See: <https://flutter.io/platform-channels/>
 class PlatformMethodChannel {
   /// Creates a [PlatformMethodChannel] with the specified [name].
   ///

packages/flutter/lib/src/services/platform_messages.dart

@@ -20,7 +20,7 @@ typedef Future<ByteData> _PlatformMessageHandler(ByteData message);
 /// * [PlatformMethodChannel], which provides higher-level platform
 /// communication such as method invocations and event streams.
 ///
-/// See: <https://flutter.io/platform-services/>
+/// See: <https://flutter.io/platform-channels/>
 class PlatformMessages {
   PlatformMessages._();