Replace deprecated withOpacity in focus_scope.0.dart‎ example (flutter#177542)

Original PR that was intended to fix all examples:  
- flutter#177205  

It was later split into smaller PRs due to a major misunderstanding and
test failures caused by color values not matching exactly.

Similar PRs:  
- flutter#177374  
- flutter#177490  
- flutter#177540  
- flutter#177541  


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Author: AbdeMohlbi

examples/api/lib/widgets/focus_scope/focus_scope.0.dart

@@ -135,7 +135,7 @@ class _FocusScopeExampleState extends State<FocusScopeExample> {
           child: Pane(
             icon: const Icon(Icons.menu),
             focusNode: foregroundNode,
-            // TRY THIS: Try changing this to Colors.green.withOpacity(0.8) to see for
+            // TRY THIS: Try changing this to Colors.green.withValues(alpha: 0.8) to see for
             // yourself that the hidden components do/don't get focus.
             backgroundColor: Colors.green,
             onPressed: backdropIsVisible ? null : () => setState(() => backdropIsVisible = true),