Fixed a link to the "One Spec to Rule them All"

In https://github.com/mongoid/mongoid/commit/01ed3f6ec6e39cd938c5b66be29a19875a679620#diff-bb1b905fa1adea594d0d1aacadce9582,

This file:
https://github.com/mongoid/mongoid/blob/master/spec/mongoid/nested_attributes_spec.rb

Was moved to:
https://github.com/mongoid/mongoid/blob/master/spec/mongoid/attributes/nested_spec.rb