Update documentation to match implementation (#108843)

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

@@ -739,7 +739,7 @@ class Image extends StatefulWidget {
   ///       child: child,
   ///     );
   ///   },
-  ///   loadingBuilder: (BuildContext context, Widget child, ImageChunkEvent loadingProgress) {
+  ///   loadingBuilder: (BuildContext context, Widget child, ImageChunkEvent? loadingProgress) {
   ///     return Center(child: child);
   ///   },
   /// )