Update docs for ActiveRecord::AttributeMethods#[]=

The docs incorrectly claimed that write_attribute is protected despite
the method being public.

activerecord/lib/active_record/attribute_methods.rb

@@ -332,7 +332,6 @@ module ActiveRecord
     end
 
     # Updates the attribute identified by <tt>attr_name</tt> with the specified +value+.
-    # (Alias for the protected #write_attribute method).
     #
     #   class Person < ActiveRecord::Base
     #   end