Fix example in serializers.md

[OttoAndrey]

Just fixed an example in Serializers docs.

[lovelydinosaur]

I guess it does make sense for the docs to be consistent with the underlying codebase here, but I also find attrs a little surprising, because validate(self, data) actually makes much more sense to me.

...So I'm feeling a little conflicted on it.

[lovelydinosaur]

Let's pass on it.