Fixed error in custom resolver documentaion.

[blafri]

There seems to be an error in the custom resolver documentaion where the CustomResolver example class inherits from Dry::Container::Registry instead of Dry::Container::Resolver, so updated the markdown in docsite to fix this.

[solnic]

Thank you and BTW - me moved dry-container to dry-core, so you can use Dry::Core::Container from now on. dry-container will be deprecated soon.

[blafri]

Thanks for the heads up on the deprecation Peter. Will start using Dry::Core::Container from now on, and thaks for all your work and the other dry-rb gem maintainers on these great libraries!!

[solnic]

@blafri sounds good and thank you 🙂