Replace mailbox doc link

Author: benoittgt

source/blog/2019-05-11-rspec-rails-4-0-has-been-released.md

@@ -185,4 +185,4 @@ Breaking Changes:
 [1]: https://relishapp.com/rspec/rspec-rails/v/4-0/docs/channel-specs
 [2]: https://guides.rubyonrails.org/action_mailbox_basics.html
 [3]: https://relishapp.com/rspec/rspec-rails/v/4-0/docs/generators
-[4]: https://relishapp.com/rspec/rspec-rails/docs/mailbox-specs/action-mailbox-spec
+[4]: https://relishapp.com/rspec/rspec-rails/v/4-0/docs/mailbox-specs