Updated Dialog.backgroundColor documentation (#71580)

packages/flutter/lib/src/material/dialog.dart

@@ -58,7 +58,7 @@ class Dialog extends StatelessWidget {
   ///
   /// This sets the [Material.color] on this [Dialog]'s [Material].
   ///
-  /// If `null`, [ThemeData.cardColor] is used.
+  /// If `null`, [ThemeData.dialogBackgroundColor] is used.
   /// {@endtemplate}
   final Color? backgroundColor;