Fix link in BlendMode.saturation (#52156)

Fix typo in the document of `BlendMode.saturation`. It has a link to image https://flutter.github.io/assets-for-api-docs/assets/dart-ui/blend_mode_hue.png, while it should be `blend_mode_saturation.png`

flutter/flutter#146820

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: PurplePolyhedron

lib/ui/painting.dart

@@ -766,7 +766,7 @@ enum BlendMode {
   /// [srcOver]. Regions that are entirely transparent in the source image take
   /// their saturation from the destination.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/blend_mode_hue.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/blend_mode_saturation.png)
   ///
   /// See also:
   ///