Update example for latest lints version (dart-lang/lints#123)

pkgs/lints/example/pubspec.yaml

diff --git a/pkgs/lints/example/pubspec.yaml b/pkgs/lints/example/pubspec.yaml
index 71ddba9..7f3ec2b 100644
--- a/pkgs/lints/example/pubspec.yaml
+++ b/pkgs/lints/example/pubspec.yaml
@@ -4,7 +4,7 @@
 publish_to: none
 
 environment:
-  sdk: '>=2.12.0 <3.0.0'
+  sdk: ^3.0.0
 
 dev_dependencies:
-  lints: ^1.0.0
+  lints: ^2.1.0