Removed "Admin App dev guide" from README

- this was originally pointing to the docs for setting up the Admin
  submodule inside Ghost but we've since switched to a monorepo so it's
  a lot easier
- the source install docs should be enough for someone to get up and
  running

README.md

@@ -80,7 +80,7 @@ Check out our [official documentation](https://ghost.org/docs/) for more informa
 
 ### Contributors & advanced developers
 
-For anyone wishing to contribute to Ghost or to hack/customize core files we recommend following our full development setup guides: [Contributor guide](https://ghost.org/docs/contributing/) • [Developer setup](https://ghost.org/docs/install/source/) • [Admin App dev guide](https://ghost.org/docs/install/source/#ghost-admin)
+For anyone wishing to contribute to Ghost or to hack/customize core files we recommend following our full development setup guides: [Contributor guide](https://ghost.org/docs/contributing/) • [Developer setup](https://ghost.org/docs/install/source/)