message: fix c# json serializer commandline argument docs
diff: @@ -155,7 +155,7 @@ To use:
  An additional feature of the object API is the ability to allow you to
  serialize & deserialize a JSON text.
  -To use Json Serialization, add `--gen-json-serializer` option to `flatc` and
  +To use Json Serialization, add `--cs-gen-json-serializer` option to `flatc` and
  add `Newtonsoft.Json` nuget package to csproj.
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cs}
  