Fixed code sample with readonly converters collection.

Partly fixes #43178.

Author: SeppPenner

docs/standard/serialization/system-text-json/snippets/how-to/csharp/RegisterConverterWithConvertersCollection.cs

@@ -13,12 +13,9 @@ public static void Run()
             // <Serialize>
             var serializeOptions = new JsonSerializerOptions
             {
-                WriteIndented = true,
-                Converters =
-                {
-                    new DateTimeOffsetJsonConverter()
-                }
+                WriteIndented = true
             };
+            serializeOptions.Converters.Add(new DateTimeOffsetJsonConverter());
 
             jsonString = JsonSerializer.Serialize(weatherForecast, serializeOptions);
             // </Serialize>