Update C#-Operations Docs

Currently Enums are not properly serialized when the standard Newtonsoft Serializer is used.

Solution:
```c-sharp
public sealed class GraphQLEnumConverter : StringEnumConverter
{
public GraphQLEnumConverter()
: base(new PascalCaseNamingStrategy())
{
}
}

internal sealed class PascalCaseNamingStrategy : NamingStrategy
{
protected override string ResolvePropertyName(string name) => name;
}

GQLClient = new GraphQLHttpClient(new GraphQLHttpClientOptions
{
EndPoint = new Uri(EndpointHttp)

}, new NewtonsoftJsonSerializer(new JsonSerializerSettings
{
Converters = new List{
new StringEnumConverter(new PascalCaseNamingStrategy())
}
}));
```

This should be added to the docs.