Fix typos in the [BottomNavigationBar] document (#67811)

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

@@ -314,10 +314,10 @@ class BottomNavigationBar extends StatefulWidget {
   /// Defaults to `12.0`.
   final double unselectedFontSize;
 
-  /// Whether the labels are shown for the selected [BottomNavigationBarItem].
+  /// Whether the labels are shown for the unselected [BottomNavigationBarItem]s.
   final bool? showUnselectedLabels;
 
-  /// Whether the labels are shown for the unselected [BottomNavigationBarItem]s.
+  /// Whether the labels are shown for the selected [BottomNavigationBarItem].
   final bool showSelectedLabels;
 
   /// The cursor for a mouse pointer when it enters or is hovering over the

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

@@ -91,12 +91,12 @@ class BottomNavigationBarThemeData with Diagnosticable {
   /// See [BottomNavigationBar.unselectedLabelStyle].
   final TextStyle? unselectedLabelStyle;
 
-  /// Whether the labels are shown for the unselected [BottomNavigationBarItem]s.
+  /// Whether the labels are shown for the selected [BottomNavigationBarItem].
   ///
   /// See [BottomNavigationBar.showSelectedLabels].
   final bool? showSelectedLabels;
 
-  /// Whether the labels are shown for the selected [BottomNavigationBarItem].
+  /// Whether the labels are shown for the unselected [BottomNavigationBarItem]s.
   ///
   /// See [BottomNavigationBar.showUnselectedLabels].
   final bool? showUnselectedLabels;