Remove superfluous `self.` in Action Mailer mail method documetation

Notation with `self` was sugestted at 0b05acd4, when self.defaults was
extlib_inheritable_accessor. But now all Action Mailer examples are
without this self. It's nicer.

actionmailer/lib/action_mailer/base.rb

@@ -596,9 +596,9 @@ def attachments
     # class method:
     #
     #  class Notifier < ActionMailer::Base
-    #    self.default from: 'no-reply@test.lindsaar.net',
-    #                 bcc: 'email_logger@test.lindsaar.net',
-    #                 reply_to: 'bounces@test.lindsaar.net'
+    #    default from: 'no-reply@test.lindsaar.net',
+    #            bcc: 'email_logger@test.lindsaar.net',
+    #            reply_to: 'bounces@test.lindsaar.net'
     #  end
     #
     # If you need other headers not listed above, you can either pass them in