[ci skip] correct doc for serialization in accordance with API Doc Guide

So here's what actually changed here:

> - You need to declare an attributes hash which contains the attributes you want to serialize.
to:
> + An +attributes+ hash needs to be declared which contains the attributes that are needed to be serialized.

I was thinking we should just say:
> An +attributes+ hash must be defined and contain any attributes you want to be serialized.