Fix a few typos in comments (#24249)

packages/flutter/lib/src/rendering/box.dart

@@ -845,7 +845,7 @@ class _IntrinsicDimensionsCacheEntry {
 /// The declaration of the `RenderFoo` class itself would thus look like this:
 ///
 /// ```dart
-/// class RenderFlex extends RenderBox with
+/// class RenderFoo extends RenderBox with
 ///   ContainerRenderObjectMixin<RenderBox, FooParentData>,
 ///   RenderBoxContainerDefaultsMixin<RenderBox, FooParentData> {
 ///   // ...

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

@@ -613,7 +613,7 @@ 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).
 ///
-/// Stateful widget are useful when the part of the user interface you are
+/// Stateful widgets 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