Fix DI example in README.md (#577)

Author: evilsquirr3l

README.md

@@ -151,8 +151,9 @@ Register the `ChatClient` as a singleton in your `Program.cs`:
 builder.Services.AddSingleton<ChatClient>(serviceProvider =>
 {
     var apiKey = Environment.GetEnvironmentVariable("OPENAI_API_KEY");
+    var model = "gpt-4o";
 
-    return new ChatClient(apiKey);
+    return new ChatClient(model, apiKey);
 });
 ```
 
@@ -1095,4 +1096,4 @@ By default, the client classes will automatically retry the following errors up
 
 ### Observability
 
-OpenAI .NET library supports experimental distributed tracing and metrics with OpenTelemetry. Check out [Observability with OpenTelemetry](./docs/observability.md) for more details.
+OpenAI .NET library supports experimental distributed tracing and metrics with OpenTelemetry. Check out [Observability with OpenTelemetry](./docs/observability.md) for more details.