Fix typo in the API docs of brightness (flutter#92713)

Author: itssunyujia

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

@@ -475,7 +475,7 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// This property is deprecated, please use [systemOverlayStyle] instead.
   ///
   /// Determines the brightness of the [SystemUiOverlayStyle]: for
-  /// [Brightness.dark], [SystemUiOverlayStyle.light] is used and fo
+  /// [Brightness.dark], [SystemUiOverlayStyle.light] is used and for
   /// [Brightness.light], [SystemUiOverlayStyle.dark] is used.
   ///
   /// If this value is null then [AppBarTheme.brightness] is used