Copy-paste error in documentation of StatefulWidget (flutter#6923)

Author: zoechi

packages/flutter/lib/src/widgets/framework.dart

@@ -502,7 +502,7 @@ abstract class StatelessWidget extends Widget {
 /// description of the user interface is fully concrete (e.g., consists
 /// entirely of [RenderObjectWidget]s, which describe concrete [RenderObject]s).
 ///
-/// Stateless widget are useful when the part of the user interface you are
+/// Stateful widget are useful when the part of the user interface you are
 /// describing can change dynamically, e.g. due to having an internal
 /// clock-driven state, or depending on some system state. For compositions that
 /// depend only on the configuration information in the object itself and the