Fix misleading "Serialized attributes" section in Upgrading Guide

At this moment this section is included in "Upgrading from Rails 4.1 to Rails 4.2". However it affects upgrading to all 4.x versions.

The change to `nil` handling in serialized attributes was actually introduced in 4.0:

* https://github.com/rails/rails/commit/39cb1bedb82814d7bf9de6834f88c31e4df27278