Correct the wrong example about JSON source generator in the readme (#1983)

Co-authored-by: Chris Pulman <chris.pulman@yahoo.com>

Author: KodamaSakuno

README.md

@@ -501,13 +501,9 @@ public class Foo
 To apply the benefits of the new [JSON source generator](https://devblogs.microsoft.com/dotnet/try-the-new-system-text-json-source-generator/) for System.Text.Json added in .NET 6, you can use `SystemTextJsonContentSerializer` with a custom instance of `RefitSettings` and `JsonSerializerOptions`:
 
 ```csharp
-var options = new JsonSerializerOptions() {
-    TypeInfoResolver = MyJsonSerializerContext.Default
-};
-
 var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com",
     new RefitSettings {
-        ContentSerializer = new SystemTextJsonContentSerializer(options)
+        ContentSerializer = new SystemTextJsonContentSerializer(MyJsonSerializerContext.Default.Options)
     });
 ```