Update README.md

Syntax highlighting for Dart code block :)
diff --git a/README.md b/README.md
index 0565d8d..405e66e 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@
 
 Example:
 
-```
+```dart
 import 'package:resource/resource.dart' show Resource;
 import 'dart:convert' show UTF8;