Update WidgetStatesController docs (#150081)

Just updated the docs that mentioned the old name (`MaterialStatesController`) that now is another class. It was showing in the [docs website](https://api.flutter.dev/flutter/widgets/WidgetStatesController-class.html) with the deprecated line-through style so I noticed it.

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

@@ -643,11 +643,11 @@ class WidgetStatePropertyAll<T> implements WidgetStateProperty<T> {
 /// or loses the focus it will [update] its controller's [value] and
 /// notify listeners of the change.
 ///
-/// When calling `setState` in a [MaterialStatesController] listener, use the
+/// When calling `setState` in a [WidgetStatesController] listener, use the
 /// [SchedulerBinding.addPostFrameCallback] to delay the call to `setState` after
 /// the frame has been rendered. It's generally prudent to use the
 /// [SchedulerBinding.addPostFrameCallback] because some of the widgets that
-/// depend on [MaterialStatesController] may call [update] in their build method.
+/// depend on [WidgetStatesController] may call [update] in their build method.
 /// In such cases, listener's that call `setState` - during the build phase - will cause
 /// an error.
 ///