ActiveModel: Fix method name in Callbacks documentation.
ActiveModel: Fix method name in Callbacks documentation.

:initializer -> :initialize

georgeclaghorn/rails,georgeclaghorn/rails,esparta/rails,georgeclaghorn/rails,MSP-Greg/rails,esparta/rails,rails/rails,rails/rails,esparta/rails,MSP-Greg/rails,esparta/rails,MSP-Greg/rails,georgeclaghorn/rails,rails/rails,MSP-Greg/rails,rails/rails