Update README.md JSON Source Generator (#1793)

JsonSerializerOptions.AddContext is obsolete. (https://learn.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializeroptions.addcontext?view=net-8.0)

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

README.md

@@ -495,8 +495,9 @@ 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();
-options.AddContext<MyJsonSerializerContext>();
+var options = new JsonSerializerOptions() {
+    TypeInfoResolver = MyJsonSerializerContext.Default
+};
 
 var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com",
     new RefitSettings {