Fix typo in Dart template: cunked -> chunked

Author: ChiragAgg5k

templates/dart/lib/src/client.dart.twig

@@ -7,7 +7,7 @@ import 'upload_progress.dart';
 
 /// [Client] that handles requests to {{spec.title | caseUcfirst}}
 abstract class Client {
-  /// The size for cunked uploads in bytes.
+  /// The size for chunked uploads in bytes.
   static const int CHUNK_SIZE = 5 * 1024 * 1024;
 
   /// Holds configuration such as project.