Typo on README.md

[Ikaro0]

All the links in the "Repo structure" had the same text while pointing to different files:

dhall/config.dhall for the config.
dhall/config.dhall for the defaults values (helps keep the config file short)
dhall/config.dhall for the Types (Types are values in Dhall)
dhall/config.dhall for the Github / Terraform specific code.

And should be:

dhall/config.dhall for the config.
dhall/default.dhall for the defaults values (helps keep the config file short)
dhall/types.dhall for the Types (Types are values in Dhall)
dhall/github.dhall for the Github / Terraform specific code.