Update example to latest stable releases (#801)

Author: kevmoo

example/README.md

@@ -5,11 +5,11 @@ dependencies to your `pubspec.yaml`.
 
 ```yaml
 dependencies:
-  json_annotation: ^3.0.0
+  json_annotation: ^4.0.0
 
 dev_dependencies:
   build_runner: ^1.0.0
-  json_serializable: ^3.2.0
+  json_serializable: ^4.0.0
 ```
 
 Annotate your code with classes defined in

example/pubspec.yaml

@@ -4,23 +4,17 @@ environment:
   sdk: '>=2.12.0-0 <3.0.0'
 
 dependencies:
-  json_annotation: ^3.0.0
+  json_annotation: ^4.0.0
 
 dev_dependencies:
   build_runner: ^1.0.0
 
   # Used by tests. Not required to use `json_serializable`.
   build_verify: ^1.0.0
 
-  json_serializable: ^3.2.0
+  json_serializable: ^4.0.0
 
   # Used by tests. Not required to use `json_serializable`.
   path: ^1.8.0
   # Used by tests. Not required to use `json_serializable`.
   test: ^1.16.0
-
-dependency_overrides:
-  json_annotation:
-    path: ../json_annotation
-  json_serializable:
-    path: ../json_serializable

example/test/readme_test.dart

@@ -25,9 +25,9 @@ void _expect(String fileName) {
 
 const _pubspecContent = r'''
 dependencies:
-  json_annotation: ^3.0.0
+  json_annotation: ^4.0.0
 
 dev_dependencies:
   build_runner: ^1.0.0
-  json_serializable: ^3.2.0
+  json_serializable: ^4.0.0
 ''';