[share] Update README.md (#3300)

* Update README.md

I just tried to update the links. The old links worked too but redirected to the current links. I just had the idea to put the current links directly.

https://pub.dartlang.org/packages/share to https://pub.dev/packages/share/

and
https://flutter.io/platform-plugins/ to https://flutter.dev/docs/development/packages-and-plugins/using-packages

* Update pubspec.yaml

* Update CHANGELOG.md

* Update CHANGELOG.md

* Update CHANGELOG.md

Co-authored-by: stuartmorgan <stuartmorgan@google.com>

packages/share/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.0.0-nullsafety.3
+
+* Update README with the new documentation urls.
+
 ## 2.0.0-nullsafety.2
 
 * Update the example app: remove the deprecated `RaisedButton` and `FlatButton` widgets.

packages/share/README.md

@@ -17,7 +17,7 @@ For more details see: https://github.com/flutter/flutter/wiki/Package-migration-
 
 ## Usage
 
-To use this plugin, add `share` as a [dependency in your pubspec.yaml file](https://flutter.dev/docs/development/platform-integration/platform-channels).
+To use this plugin, add `share` as a [dependency in your pubspec.yaml file](https://flutter.dev/docs/development/packages-and-plugins/using-packages/).
 
 ## Example
 
@@ -44,4 +44,4 @@ To share one or multiple files invoke the static `shareFiles` method anywhere in
 ``` dart
 Share.shareFiles(['${directory.path}/image.jpg'], text: 'Great picture');
 Share.shareFiles(['${directory.path}/image1.jpg', '${directory.path}/image2.jpg']);
-```
+```

packages/share/pubspec.yaml

@@ -2,7 +2,7 @@ name: share
 description: Flutter plugin for sharing content via the platform share UI, using
   the ACTION_SEND intent on Android and UIActivityViewController on iOS.
 homepage: https://github.com/flutter/plugins/tree/master/packages/share
-version: 2.0.0-nullsafety.2
+version: 2.0.0-nullsafety.3
 
 flutter:
   plugin: